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Alternaria and TomCast

Alternaria solani occasionally causes leaf spot on pepper foliage. Alternaria alternata may cause fruit rot, particularly following sun scald or blossom end rot. Sun scald on pepper fruit usually occurs when the foliage is sparse and the peppers are exposed to sunlight. The injury becomes tan and shrunken and may appear water-soaked. When Alternaria colonizes these lesions they become chocolate brown to black and the fungus may be evident by a felty, dark brown to black growth. Alternaria fruit rot may also occur post-harvest.

The fungus infects stems, leaves and fruits. It may girdle seedlings causing damping-off in the seedbed. On the leaves, brown circular spots are often surrounded by a yellow area. Leaf spots have characteristic dark concentric rings. Leaf spots usually appear on the oldest leaves first and progress up the plant. As the disease progresses, the fungus may infect the stems and fruit. The spots on the fruit look similar to those on the leaves--brown with dark concentric rings. Dark, dusty spores are produced in concentric rings. The spores can be seen if the spot is touched to a light-colored object.

The fungus can survive in soil and in infested crop and weed residues. It may be seed-borne and carried by wind, water, insects, workers and farm equipment. The spores that land on  plants will germinate and infect the leaves when they are wet. Spores can enter the leaf, steam or fruit. The fungus is most active during mild to warm temperatures and wet weather. The disease is worse during the rainy season. Early blight is most severe on plants stressed by a heavy fruit load, nematode attack, or low nitrogen fertility.

The susceptibility of the most pepper varieties to Alternaria is very low. Therefore using the TomCast model needs to adopt the action treshold to the specific susceptibility of the cultivar.  Eggplant is susceptible to Alternaria. For this plant the tresholds, originally used in tomato are more propriate.

Model Tom Cast

The TomCast model is designed to evaluate the data of the first needed spray and the needed spray intersept. It is calculated on base of hours with leaf wetness (or relative humidtiy more than 90%) and the average temperature during this period. Every day is evaluated for this and every day gets a severity figure in between 0 and 4.

To assess the date of the first spary and the date when a spray has to be repeated the accumulated severity values are used. On the normally low susceptible pepper cultivars a high value (40 and more) can be accepted. After an injury like hail susceptibility is much higher and the acculated severity values have to be reduced to 18 to 25. This treshold has to be applied in Eggplants too.

Output in Fieldclimate.com: Severity values are accumulated regarding to favourable conditions of leaf wetness and temperature within this period (see tabel above). In Sum 24 Severity values have been accumulated within the observed time span of 10th of July to 27nd of August. In dependence of the cultivar (susceptibility) and of injuries the severity value a spray application is recommended at about 40 SV (not susceptible cultivar, no injuries) or at about 20 SV, when injuries are observed through hail or the cultivar is very susceptible to the pathogen.