• POTATO LATE BLIGHT – Phytophthora infestans

• TomCast ALTERNARIA MODEL FOR POTATO

• COLORADO POTATO BEETLE

• APHID RISK MODEL

POTATO LATE BLIGHT – Phytophthora infestans

Potato Late Blight caused by Phytophtora infestans is one of the most devastating plant diseases. It has led to starvings and emigration when it came to Europe. It is one of the most important diseases and therefore numerous models are available for it. P. infestans is an obligate parasite. It can only live in the green tissue of its hosts. The economic important plants under its hosts are potato, tomato and egg plant. In the cool climate during winter the pathogen will find no green tissue and it has to hibernate in infected tubers or in its fruiting bodies the oospores. Oospores will only be formed in places where two different mating types of P. infestans are present. This is reported for Europe since the last 25 years. Still of greater importance is the hibernation in infected tubers left as volunteers on the field because of undersize or other reasons or damped on the field as waste form potato storage.

Newer laboratory methods enabled us to check for latent infected tubers in the potato seed. This showed that we have to expect this in the potato seed. The quantities with which we have to expect latent infected seeds is depending on the blight epidemics of the last season in the seed producing area.

P. infestans grows like other oomycetes in the intercellular area of its hosts. Systemic growth is enforced by high relative humidity and by high soil water content or low soil oxygen content. Plants formed by latent or symptomatically infected tubers show extended systemic growth in periods with water logging. In the morning during and after such periods you will find potato sprouts covered by white sporangia. Sporangia in oomycetes are formed in the absence of light if relative humidity is high and temperature are high enough. For P. infestans sporangia formation will take place in nights with relative humidity higher than 90% and temperatures warmer than 10°C. Sporangia can be distributed by rain or wind.

In literature we can find informations about sporangia germinating and infecting like conidia. Sporangia in oomycetes usually germinate with zoospores which are mobile in free water. The zoospores are swimming to stoma trough which its infects its host. Jim Deacon from Institute of Cell and Molecular Biology, The University of Edinburgh found that at temperatures of 12°C and less the majority of sporangia release zoospres whereas at temperatures lighter than 20°C the majority of the sporangia germinates like conidia with germ tubes. Therefore infection of P. infestans in cool climate is most likely limited by the presence of free moisture which can be given by dew in nights which have more than 90% relative humidity needed for sporangia formation. More severe infections have to be expected with rain distributing the zoospores over the potato field and leading to an exponential increase in infected plants.

In heavily infected plants the pathogen will grow systemic into all plant organs including the tubers. In situations with severe disease pressure the potato leaf has to be killed with herbicide to avoid the infections of the tuber.

Although Fieldclimate.com is supports multiple models for late blight prediction, we recommend the use of 3 models for this disease.
A simple rule to predict the first spray: When you could not enter into your potato field for 3 days due to extended rains, start to spray immediately, if possible using a curative compound.
Use the Phytophthora infestans Infection Prediction model to confirm the possible infection dates.
Use NoBlight model to define spraying with preventive fungicides.

The Negative Prognosis Model by Schrödter and Ullrich

Negative prognosis means NOT to spray as long the prognosis answers the question about the presence of the pathogen in field with NO. This explains the term negative prognosis. The Schrödter and Ullrich Negativ prognosis have been published in the year 1972. It uses temperature, leaf wetness or high relative humidity and rain to assess the propagation of the pathogen in the potato field.

Modelling the infection by Pythophthora infestans

A value in between 0 and 400 is indicating the propagation of P. infestans in the field. This value increases if air temperature is in between 15°C and 20°C, if relative humidity is higher then 70%. It increases faster for all times if relative humidity is higher than 90% and there is precipitation or if there is leaf wetness for more than 4 hours. If this situation lasts longer than 10 hours the increase is higher. Whereas the original model defines the start of calculation with the emergence of the potato in the specific field, we changed the start of calculation to a temperature based rule making sure that we calculate as soon as the first possible potato will grow. For potato we will calculate as soon as temperature from 10:00 to 18:00 is higher than 8°C and night temperature is never below 2°C.

Interpretation of results in FieldClimate

Schrödter and Ullrich are defining a value of 150 to correspond with a disease incidence in the field of 0.1%. A value of 250 corresponds with a disease incidence of 1%. They suggest that after a year with a low pressure of late blight in the seed producing area no sprays are needed before a value of 250 is reached. If a higher amount of inoculum has to be assumed sprays should start at 150. The negative prognosis has been used very successful starting form 1972 until the nineties of the last century. This has been the time before we could find resistance against Metalaxyl. The first spray in these years was usually done by Metalaxyl and with this the field could be cleared up from P. infestans. Now big areas have resistance against this compound and we do not have any fungicide showing a similar clearing up effect. In areas where covered potato is grown beside of open field potato we suggest to start spraying as soon as the plastic is removed from the covered crop. The disease can develop under the plastic and the covered crop will become an inoculum source after uncover.

P. infestans grows systemic inside the sprout of young potato. This is important if we have latent infected potato seed. The systemic growth is much favoured by water over saturated soil. To be able to receive information over water saturation of the soil we suggest the use of watermark sensors. Watermarks are very economic and very helpful for potato irrigation. If we have a period of several hours after emergence where the water tension of the watermark sensor is below 10 cBar (100mBar) and more than 10°C air temperature we have to assume good conditions for systemic growth of the pathogen and we have to start with the sprays against late blight. The graph shows an increasing infection by P. infestans reaching the value of 150 on the 6th of June (Negativ Prognose Stufe) and the value 250 on the 26nd of June (= Negativ Prognose Stufe, green line). Protection measurements should be taken into account in dependance of the history (inoculum, pressure of late blight the last year).

The Late Blight Infection Model of FRY

Sensors needed: Precipitation, Leaf Wetness, relative Humidity and Temperature

W.E.FRY (1983) published his work done on infection of potatoes with different susceptibility levels at different durations of relative humidity higher than 90% or leaf wetness and temperatures. Derived from these results he developed an infection model for late blight in potato and in the next step a model to estimate the appropriate spray interval for the fungicide cloranthonil (Bravo).

Susceptible cultivars can be infected within shorter moist periods and the disease severity will be higher. Whereas moderate susceptible and resistant varieties will need a longer moist period or warmer temperatures to be infected and the disease severity is lower.

For susceptible varieties the maximum rating of an infection period can be 7 whereas for moderate susceptible varieties it can be 6 and for resistant varieties it can be 5 only. In the same way the assessment of the spray interval is asking again for the level of susceptibility of the cultivar. A spray is needed if the last spray is longer than 6 days away and the accumulated blight Units are exceeding: 30 for susceptible varieties, 35 for moderate susceptible varieties and 40 for moderate resistant varieties. This model can be found to be referred as SIM. The SIM model can be used to estimate the first spray too. A first spray would be appropriate if the emergence the thresholds of 30, 35 or 40 accumulated disease severity values are exceeded. This model can be applied in areas with continuous potato or tomato growing too.

This model is very useful to estimate if a new spray is needed. We can start to accumulate the Fry units from the date of the last spray on. If the accumulated value excites the threshold we will have to spray again.

In FieldClimate.com the infections of the three severity classes of Susceptible, Moderate and Resistent potato varieties are displayed by an infection curve. When 100% infection have been reached the conditions for infection by P. infestans have been optimal. In this example we see good conditions for infections on the beginning of May, but varieties (especially moderate and resistent ones) would not have been infected because hours of high relative humidity have been too short.

Literature:
Fry, WE, AE Apple & JA Bruhn (1983). Evaluation of potato late blight forecasts modified to incorporate host resistance and fungicide weathering. Phytopathology 73:1054-1059.
Fry, WE, AE Apple & JA Bruhn (1983). Evaluation of potato late blight forecasts modified to incorporate host resistance and fungicide weathering. Phytopathology 73:1054-1059.

The Combination of Negative Prognosis and Fry Infection

We combined the negative prognosis model following Schrödter and Ullrich with the assessment for the spray interval by the FRY model this is called NegFry. This combination is used in Denmark and Northern Europe with success.

The negative prognosis model defines the date of the first spray in dependence of the last year pressure a threshold of 150 or 250 is used for the first spray against late blight. This first spray can be still made with a Metalaxyl containing product, knowing that with the first and only application of Metalaxyl we can expect efficacies of 75% to 80%. All the next sprays will be done with preventative products. This can be Mancozep or Chlorthalonil.

In Netherlands and Belgium there have been discussions not to use Metalaxyl containing products at all. In this case the use of the negative prognosis to estimate the date of the first spray might be problematic. As alternative solutions we would suggest in areas with covered early potatoes to start spray as soon as the plastic is removed from the early potatoes. In areas without early potato we suggest the use of watermark sensor to sense for water logging situation. As soon as we have an ambient temperature higher than 10°C and water tensions smaller than 10 cBar (100 mBar) for several hours we have to expect systemic growth of the pathogen starting from latent infected seed tubers. The sprouts of this potatoes will become covered by sporangia over night and epidemic will start with force. After the first water logging situation at temperatures higher than 10°C we have to start the preventative spray program.

NoBlight Model

Late Blight Prediction in Maine – used to guide the initiation and subsequent applications of fungicides for control of potato late blight developed by Steven B. Johnson, Extension crops specialist, UNIVERSITY OF MAINE COOPERATIVE EXTENSION.

Sensors needed: Precipitation, relative humidity and temperature

Late blight control in Maine depends on proper application—timing, rate, and coverage—of protectant materials. The use of predictive models can permit late blight control with fewer, timelier chemical applications, which will help control costs and reduce chemical inputs to the environment.

Assessing the potential for late blight: Fungicide applications to control late blight should be based on weather conditions, not on a calendar. In most years, a calendar-based program applying fungicides weekly may start fungicide applications earlier than needed. In many years, portions of the growing season may need fungicide applications more frequently than once per week, while other portions of the growing season may need fungicide applications less frequently than once per week. Application of late blight control materials should be based on a predictive model in order to be efficient and effective.

In Maine, the potential for late blight to appear is predicted with severity values. Severity values are based on weather conditions and accumulate when they are appropriate for the development of the pathogen. The environmental conditions conducive to late blight development are generally mild and wet.

Difference between NoBlight and Blitecast

“Blitecast,” (a form of NoBlight model), which uses Wallin’s model of severity value accumulation. Wallin severity values are derived from various combinations of the hours with a relative humidity of 90 percent or greater and the average temperature during those periods. The duration of continuous periods of relative humidity of 90 percent or greater is tracked and the average temperature during these periods is calculated. Severity values are assigned based on these measurements and calculations and are accumulated. The first occurrence of late blight is predicted seven to ten days after 18 severity values have accumulated. The NoBlight model initiates accumulation of severity values starting at 50 percent plant emergence.

NoBlight like Blitecast, weights relative humidity more heavily than rainfall in predicting the timing of the applications. The spray interval becomes shorter with the accumulation of 25 mm (1.18 inches) of rain over the previous seven days under the same number of accumulated severity values. NoBlight differs from Blitecast in the accumulation of severity values based on relative humidity. NoBlight does not stop accumulating conducive conditions where the relative humidity drops below 90 percent. Blitecast uses 76.5 percent relative humidity to discontinue accumulation of conducive infection conditions.

Usually, this adds a half hour or more onto the typical Wallin hours. Typically this is a dewy morning period in Maine summers. More importantly, this does not discontinue the accumulation of conducive conditions when the relative humidity drops to 88 percent for a period of time. In effect, the severity values accumulated by NoBlight are more conservative that the Wallin severity values. Three separate six-hour periods of relative humidity greater than 90 percent will not accumulate any severity values.

However, an 18-hour period of relative humidity greater than 90 percent will accumulate severity values, depending on the average temperature during that period (3 severity values at 18.3 °C (65°F), 2 at 13.3 °C (56°F), 1 at 10 °C (50°F), and 0 at 4.4 °C (40°F) or 29.4 °C (85°F)). Once 18 severity values have accumulated after emergence, a protective fungicide application is recommended. After that time, the recommended application interval is based on additional severity value accumulation during the previous seven days in the manner described in Table 2. Fungicide treatment for the prevention of late blight should begin immediately if the disease is developing from seed or has otherwise been sighted in the field or nearby fields.

As with any model, NoBlight is no better than the data it analyses. The value of a predictive model is to provide the user with a reliable estimate of when conditions are conducive for late blight development and when conditions are not conducive for late blight development. The model provides some guidance on when a grower can stretch spray intervals with minimal risk, as well as when the spray interval needs to be reduced because the crop is at risk.

Smith Periods to Predict Potato Late Blight

Sensors needed: Air temperature, relative Humidity

Biological Basis of the model: Phytophtora infestans can grow if temperature is lower than 10°C. But sporulation will be nearly nothing at this temperatures. Therefore it needs a moist period with temperatures higher than 10°C to get a reasonable sporulation. Infection of Phytophtora infestans needs free moisture. In longer periods of high relative humidity free moisture either by rain or by dew is very much probable.

What is a Smith Period? Two consecutive days with minimum temperature of 10 °C and 10 hours of relative humidity higher than 90% on the first day and 11 hours of relative humidity higher than 90% on the second day is a Smith Period. If the criteria for the fist day is fulfilled and the second day reaches 10 hours of relative humidity higher than 90% this indicates that 90% of the Smith period or Near Smith.

Interpretation

Smith periods or near Smith periods are pointing out periods where the climate is very favourable for the disease. The model points out periods with a very high risk of this disease. Experience: This is an empirical model showing very good results in UK where it is used as a negative prognosis too. As long it is to cold for 2 moist days with temperature always higher than 10°C no spray is needed. This model is only valid where temperature increase during spring is very steady (Ocean Climate).

Literature:
Smith, L. P. 1956. Potato blight forecasting by 90% humidity criteria. Plant Pathology 5:83-87 (Basic model).
Hims, M. J., M. C. Taylor, R. F. Leach, N. J. Bradshaw, and N.V. Hardwick, 1995. Field testing of blight risk prediction models by remote data collection using cellphone analogue networks, p. 220-225 In: Phytophthora infestans 150: European Association for Potato Research (EAPR)-Pathology Section Conference, held in Trinity College, Dublin, Ireland, September 1995 to mark the one hundred and fiftieth anniversary of the first record of potato blight in Ireland and the subsequent famine. L. J. Dowley, et al. (Eds). Boole Press, Ltd. Dublin. pp. 220-225.

Model WinstelCast for P. infestans

Input variables:
Environmental: Temperature, relative humidity.
Calculated: Daily average, minimum and maximum temperatures, hours of temperatures greater than 10°C and relative humidity greater than 90%.

This model is composed of two phases. Phase 1 predicts infection, which is predicted after the following requirements are met: After the daily average temperature is between 10°C and 23° C and then 10 hours or more of temperatures greater than 10° C and relative humidity greater than 90% occur (such periods are considered to be the same as leaf wetness). Phase 2 sets criteria for pathogen growth. Phase 2 occurs when the maximum daily temperature for two consecutive days is between 23°C and 30°C. Phase 2 must occur at least 24 hours but not later than 10 days after phase 1.

Treatment should be initiated, when phase 1 occurs and is followed by phase 2. Be aware that this model was developed for early potato varieties!

Literature:
Developed by Winstel, K. 1993. Kraut- und Knollenfaule der Kartoffel eine neue Prognosemoglichkeit-sowie Bekämpfungsstrategien. Med. Fac. Landbouww. Univ. Gent, 58/3b.

Model BliteCast for P. infestans

Sensor needed: Precipitation, temperature, relative humidity, leaf wetness

BLITECAST is used to model the first possible infection by P. infestans
BLITECAST is an integrated computerized version of both the Hyre and the Wallin model. The first part of the program forecasts the initial occurrence of late blight 7-14 days after the first accumulation of 10 rain-favorable days according to Hyre’s criteria, or the accumulation of 18 severity values according to Wallin’s model. The second part of the program recommends fungicide sprays based on the number of rain-favorable days and severity values accumulated during the previous seven days. Accumulation of rain-favorable days and severity values begins when distinct green rows can be seen in the potato field, and ends at vine kill. The first spray is recommended when the first late blight forecast is given. Subsequent sprays are recommended according to an adjustable matrix which correlates rain-favorable days with severity values.

Threshold for applications

First spray is recommended when the first forecast is given. Subsequent treatments are based on the following table:

Adjustable matrix used to relate severity values and rain-favorable days and generate spray recommendation for Blitecast.

Model description

Severity Value Accumulation Using Wallin’s System of Forecasting Late Blight (Blitecast) Hours of RH > 90%

Average temperature during period of relative humidity (RH) has to be 90% or greater.
Late blight is first expected to appear no earlier than within 1-2 weeks after 18 SV have accumulated starting at the time of first emergence of green tissue from the source of late blight inoculum. The source of inoculum could be plants growing from infected tubers in a cull pile, volunteers growing from infected tubers that survived the winter, or infected seed tubers. The first green tissue is most likely to be emerging from any potato cull piles in your area, so it’s best to use that date.

Irrigation* can create late blight favorable conditions in a field that a weather monitor will not be taking into account. Irrigation that starts when the leaves are still wet from dew in the morning, or continues after dew has fallen at night will extend the wetting period for that day.

Literature:
Page is refered to http://www.ipm.ucdavis.edu/DISEASE/DATABASE/potatolateblight.html
Krause, R. A., Massie, L. B., and Hyre, R. A. 1975. BLITECAST, a computerized forecast of potato late blight. Plant Disease Reporter 59: 95-98. MacKenzie, D. R. 1981. Scheduling fungicide applications for potato late blight. Plant Disease 65: 394-399. MacKenzie, D. R. 1984. Blitecast in retrospect a look at what we learned. FAO Plant Protection Bulletin 32:45-49.

Model « Phytophtora infestans »

Calculation of sporulation start at night with relative humidity above 80%. If sporulation takes place and it is raining the infection starts to be calculated at air temperatures between 10 and 30 °C.

The calculation for sporulation stops if solar radiation is above 700, and relative humidity below 40.

The calculation for infection stops if relative humidity falls below 80%.

Severity values are calculated from 0 to 5 (if infection has been determined) with 0: very low pressure and 5: high pressure.

Pectobacterium « Blackleg » Disease

---

TomCast ALTERNARIA MODEL FOR POTATO

The dark coloured spores and mycelium of the pathogen survive between growing seasons in infested plant debris and soil, in infected potato tubers and in overwintering debris of susceptible solanaceous crops and weeds including hairy nightshade (Solanum sarrachoides). Overwintering spores and mycelia of A. solani are melanized (darkly pigmented) and can withstand a wide range of environmental conditions including exposure to sunlight and repeated cycles of drying, freezing and thawing. In spring, spores (conidia) serve as primary inocula to initiate disease. Plants grown in fields or adjacent to fields where potatoes were infected with early blight during the previous season are most prone to infection, since large quantities of overwintering inoculum are likely to be present from the previous crop. Initial inoculum is readily moved within and between fields, as the spores are easily carried by air currents, windblown soil particles, splashing rain and irrigation water.

Spores of A. solani are produced on potato plants and plant debris between 5°C and 30°C (the optimum is 20°C). Alternating wet and dry periods with temperatures in this range favour spore production. Few spores are produced on plant tissue that is continuously wet or dry. The dissemination of inoculum follows a diurnal pattern in which the number of airborne spores increases as leaves that are wet with dew or other sources of nighttime moisture dry off, relative humidity decreases and wind speeds increase. The number of airborne spores generally peaks in mid-morning and declines in late afternoon and at night.

Spores landing on leaves of susceptible plants germinate and may penetrate tissues directly through the epidermis, through stomata and or through wounds such as those caused by sand abrasion, mechanical injury or insect feeding. Free moisture (from rain, irrigation, fog or dew) and favourable temperatures (20-30°C) are required for spore germination and infection of plant tissues. Lesions begin to form 2 to 3 days after initial infection.

Many cycles of early blight spore production and lesion formation occur within a single growing season once primary infections are initiated. Secondary spread of the pathogen begins when spores are produced on foliar lesions and carried to neighbouring leaves and plants. Early blight is largely a disease of older plant tissues and is more prevalent on senescent tissues on plants that have been subjected to stresses induced by injury, poor nutrition, insect damage, or other types of stress. Early in the growing season the disease develops first on fully expanded leaves near the soil surface and progresses slowly on juvenile tissues near the growing point. The rate of disease spread increases after flowering and can be quite rapid later in the season during the bulking period and during periods of plant stress. Early blight lesions are often found on most leaves of unprotected plants late in the growing season.

In potato tubers, germinated spores penetrate the tuber epidermis through lenticels and mechanical injuries to the skin. Tubers often become contaminated with A. solani spores during harvest.These spores may have accumulated on the soil surface or may have been dislodged from desiccated vines during harvest. Infection is most common on immature tubers and those of white- and red-skinned cultivars, since they are highly susceptible to abrasion and skinning during harvest. Course-textured soil and wet harvest conditions also favour infection. In storage, individual lesions may continue to develop but secondary spread does not occur. Infected tubers may shrivel through excessive water loss, depending on storage conditions and disease severity. Early blight lesions on tubers, unlike late blight lesions, are usually not sites of secondary infection by other decay organisms.

Model TomCast

developed by Jim Jasinski, TOMCAST Coordinator FOR OHIO, INDIANA, & MICHIGAN

Background: TOMCAST (TOMato disease foreCASTing) is a computer model based on field data that attempts to predict fungal disease development, namely Early Blight, Septoria Leaf Spot and Anthracnose on tomatoes. Field placed data loggers are recording hourly leaf wetness and temperature data. This data where analyzed over a 24 hour period and may result in the formation of a Disease Severity Value (DSV); essentially an increment of disease development. As DSV accumulate, disease pressure continues to build on the crop. When the number of accumulated DSV exceed the spray interval, a fungicide application is recommended to relieve the disease pressure.

TOMCAST is derived from the original F.A.S.T. (Forecasting Alternaria solani on Tomatoes) model developed by Drs. Madden, Pennypacker, and MacNab at Pennsylvania State University (PSU). The PSU F.A.S.T. model was further modified by Dr. Pitblado at the Ridgetown College in Ontario into what we now recognise as the TOMCAST model used by Ohio State University Extension.

DSVs are: A Disease Severity Value (DSV) is the unit of measure given to a specific increment of disease (early blight) development.

In other words, a DSV is a numerical representation of how fast or slow disease (early blight) is accumulating. The DSV is determined by two factors; leaf wetness and temperature during the « leaf wet » hours. As the number of leaf wet hours and temperature increases, DSV accumulate at a faster rate. See the Disease Severity Value Chart below.

Conversely, when there are fewer leaf wet hours and the temperature is lower, DSV accumulate slowly if at all. When the total number of accumulated DSV exceeds a present limit, called the spray interval or threshold, a fungicide spray is recommended to protect the foliage and fruit from disease development.

The spray interval (which determines when you should spray) can range between 15-20 DSV. The exact DSV a grower should use is usually supplied by the processor and depends on the fruit quality and end use of the tomatoes. Following a 15 DSV spray interval is a conservative use of the TOMCAST system, meaning you will spray more often than a grower who uses a 19 DSV spray interval with the TOMCAST system. The trade off is in the number of sprays applied during the season and the potential for difference in fruit quality.
USING TOMCAST: Potatos grown within 10 miles of a reporting station should benefit from the disease management function of TOMCAST to help forecast Early blight, Septoria, and Anthracnose.

If you decide to try TOMCAST this season please keep in mind three very important concepts:

1) If this is your first time using the system, it is recommended that only part of your acreage be put into the program to see how it fits with your quality standards and operational style.

2) Use TOMCAST as a guide to help better time fungicide applications, realizing in some seasons you may actually apply more product than a set schedule program might require.

3) The further a field is from a reporting site increases the likelihood of distortion in the DSV accumulation, i.e., the reported value may be a few DSV higher or lower than that experienced by the field location. This should be taken into consideration when application of fungicides is likely a few days away. Listen to the DSV reports of nearby stations and triangulate to your own location as the best way to roughly estimate your DSV accumulation.
FIRST SPRAY USING TOMCAST: There has been some discussion over the years regarding the application of the first spray when following TOMCAST. The rule stated in the 1997 Vegetable Production Guide centers around the planting date.

A)

Tomato plants that enter the field before May 20 should have the first spray applied when DSV for that area exceed 25 or when a fail safe date of June 15 arrives. The fail safe is used only if you have not treated since May 20, and is a means to eliminate initial disease inoculum. After the first spray, these tomatoes are subsequently treated when the chosen spray interval (range 15-20 DSV) is exceeded.
Tomatoes planted after May 20 are treated when they exceed the chosen spray interval (range 15-20 DSV) or when they have not been treated by the fail safe date of June 15. Therefore, it is critical to compare the tomato planting date to the date DSV reporting began in that area to guide the spray decision process.)

B)

The first fungicide application for early blight occurs once cumulative P-Days after emergence reach 300.

Physiological Day (P-Day).
The P-Day procedure was proposed by Sands et al. (1979) to predict potato yield and modified by Pscheidt and Stevenson (1986) for application to potato development and early blight appearance. The P-Day calculation requires only daily maximum and minimum temperatures as input. The algorithm is: 8 P-Days ={1/245P(Tmin) + 8P(2Tmin/3 + Tmax/3) + 8P(2Tmax/3 + Tmin/3) + 3P(Tmax)}

Where:

P(T) = 0 if T < 7°CP(T) = 101 – (T – 21)2 /(21 – 7)2 if 7°C < T < 21°CP(T) = 101 – (T – 21) 2 /(30 – 21) 2 if 21°C < T < 30°C starting at emergence.P(T) = 0 if T >30°C Tmin – minimum daily temperature (°C) Tmax – maximum daily temperature (°C)

The model assumes 7°C minimum, 21°C optimum and 30°C maximum growth temperatures for potato plant development, as well as diurnal fluctuations.

Growing Degree Day
The Growing Degree Day (GDD) method was modified by Franc et al. (1988) for initiation of fungicide applications to control early blight in Colorado.

The proposed base temperature of 7.2° C resulted in the subsequent equation:

((Tmax/Tmin)/2)+7,2

They reported that primary lesions could be expected to appear at cumulative 361 GDD in the San Luis Valley area of Colorado, whereas primary lesions would only be expected to appear after 625 GDD in northeastern Colorado.

Although it was developed to predict early blight, septoria leaf spot, and anthracnose development on tomatoes, the model has been used successfully to predict early b light development on potatoes (Pscheidt and Stevenson, 1988; Christ and Maczuga, 1989).

---

COLORADO POTATO BEETLE

Colorado potato beetle (Leptinotarsa decemlineata) is the most important insect defoliator of potatoes. It also causes significant damage to tomato and eggplant. One beetle consumes approximately 40 cm2 of potato leaves at a larval stage, and up to additional 9.65 cm2 of foliage per day as an adult (Ferro et al., 1985). In addition to impressive feeding rates, Colorado potato beetle is also characterized by high fecundity, with one female laying 300-800 eggs (Harcourt, 1971). Furthermore, the beetle has a remarkable ability to develop resistance to virtually every chemical that has ever been used against it.

Distribution

Since Colorado potato beetle shifted from its original wild hosts in southwestern North America, it has spread throughout the rest of the continent and has invaded Europe and Asia. Currently its distribution covers about 8 million km2 in North America (Hsiao, 1985) and about 6 million km2 in Europe and Asia (Jolivet, 1991). It has appeared recently in western China and Iran. Potentially the Colorado potato beetle can occupy much larger areas in China and Asia Minor, spread to Korea, Japan, Russian Siberia, certain areas of the Indian subcontinent, parts of North Africa, and the temperate Southern Hemisphere (Vlasova, 1978; Worner, 1988; Jolivet, 1991).

History

The Colorado potato beetle has a complicated and diverse life history. The beetles overwinter in the soil as adults, with the majority aggregating in woody areas adjacent to fields where they have spent the previous summer (Weber and Ferro, 1993). The emergence of post-diapause beetles is more or less synchronized with potatoes. If fields are not rotated, they are colonized by overwintered adults that walk to the field from their overwintering sites or emerge from the soil within the field (Voss and Ferro, 1990). If fields are rotated, the beetles are able to fly up to several kilometers to find a new host habitat (Ferro et al., 1991; 1999). Once they have colonized the field, the overwintered beetles first feed and then oviposit within 5-6 days depending on temperature (Ferro et al., 1985; Ferro et al., 1991).

Eggs are usually laid on the underside of potato leaves. Upon hatching, larvae may move over short distances within potato canopy and start feeding within 24 hours of eclosion. Development from the time of oviposition to adult eclosion for pupae takes between 14-56 days (de Wilde, 1948; Walgenback and Wyman, 1984; Logan et al., 1985; Ferro et al., 1985). The optimal temperatures range between 25-32ºC and appear to differ among populations of different geographic origins. The larvae are capable of behavioral thermoregulation via moving within plant canopies (May, 1981; Lactin and Holliday, 1994), thus optimizing their body temperature compared to the ambient temperature. Pupation takes place in the soil near the plants where the larval development has been completed.

Diapause is facultative, and the beetles can have between one and three overlapping generations per year. It takes a few days for the newly emerged adults to develop their reproductive system and flight muscles (Alyokhin and Ferro, 1999). After development has been completed, the beetles mate and start laying eggs. The reproduction continues until diapause is induced by the short-day photoperiod, then the beetles migrate to overwintering sites (mainly by flying), and enter the soil to diapause. Those beetles that emerge under short-day photoperiod do not develop their reproductive system and flight muscles that season. They feed actively for several weeks and then either walk to the overwintering sites or burrow into the soil directly in the field (Voss, 1989).

Colorado potato beetle’s diverse and flexible life history is well-suited to unstable agricultural environments, and makes it a complex and challenging pest to control. Flight migrations closely connected with diapause, feeding and reproduction allow the Colorado potato beetle to employ « bet-hedging » reproductive strategies, distributing its offspring in both space (within and between fields) and time (within and between years). Such strategies minimize the risk of catastrophic losses of offspring, otherwise quite possible in unstable agricultural ecosystems (Solbreck, 1978; Voss and Ferro, 1990).

Source: http://www.potatobeetle.org/overview.html

Model of Colorado Potato Beetle

Risk model For the calculation of the occurrence of the Colorado potato beetle we take into consideration: x) the duration of sunlight of the day (14 hours or 15hours of sunlight)
x) Soil temperature above 12°C
x) Average air temperatures during the last four days in combination with the day length give a value from 1- 4 (severity): 1= very low risk of Colorado potato beetle 2= low risk of Colorado potato beetle 3= average risk of Colorado potato beetle 4= high risk of Colorado potato beetle.

FieldClimate

The calculation of the risk is based on the determination of the soil temperature and air temperature during a time period of the last 4 days. Soil temperature has to be above 12°C and in sum about 100800 degreeminutes (soil temperature * time) have to be reached to lead to beetle occurrence (basic condition for occurrence). Different severity classes are figured out (from 1- to 4, see above). On the graph you see that till the beginning of June the risk was 0 or very low. At the beginning of June conditions for the occurrence of the Colorado beetle (more than 14/ 15hours sunlight and average air temperatures of 20-23 °C) have been good and a severity of 3, which means a moderate risk was determined.

---

APHID RISK MODEL

Conditions: In the morning when sun raises and relative humidity decreases, optimum temperatures between 20°C and 32°C – good flight is indicated.

If temperatures are not in the optimum range (to cold/hot) or it is too wet (leaf wetness) risk decreases.

Output is the daily risk.

So optimum temperatures and falling relative humidities during the morning are indicating a good flight day. When it is wet during the night and temperatures are to low this is bad for propagation. The same when it is hot and moist during the day.