TomCast Alternaria Model for Potato


Early blight caused by the fungus Alternaria solani.

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.

Earlyblight Fig 1 1 Earlyblight Fig 4 1 Earlyblight Fig 3 1

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.


( 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 recognize as the TOMCAST model used by Ohio State University Extension.

TomCasttabelle  DSV 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.


(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.) 


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)}


P(T) = 0 if T < 7°C

P(T) = 101 – (T – 21)2 /(21 – 7)2 if 7°C < T < 21°C

P(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:

They reported that primary lesions could be expected to appear at cumulative 361 GDD in the San Luis Valley area of Colorado, wher eas 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 succe ssfully to predict early b light development on potatoes (Pscheidt and Stevenson, 1988; Christ and Maczuga, 1989).