Onion Stemphylium Leaf Blight
Stemphylium leaf blight is caused by the fungus Stemphylium vesicarium. Small, light yellow to brown and water-soaked lesions develop on leaves. These small lesions grow into elongated spots that frequently coalesce resulting in blighted leaves. Lesions usually turn light brown to tan at the center and later dark olive brown to black as the spores of this pathogen develop. S. vesicarium normally invades dead and dying onion tissue, such as leaf tips, purple blotch and downy mildew lesions, injured tissue, and senescent tissue. Infection usually remains restricted to leaves and does not extend into the bulb scales. Lesions generally occur on the side of the leaf facing the prevailing wind. Long periods of warm wet conditions encourage disease development.
Stemphylium versicarium model
is based on the work of SUHERI and PRICE in onions and LLorent, VILARDELL, BUGIANI, GHERARDI and MONTESINO in pear. Infection curves for weak, moderate and severe infections are computed. It depends on the disease history of the location wheater a treatment has to be taken into consideration by an weak, moderate or heavy infection.
With this separation into favourable, moderate favourable and very favourable situations it is to the grower to decide how big the disease pressure in a specific field will be and if he has to cover a specific infection.
Leaf wetness > 0 (start for infection) or rel. humidtiy > 90% (just if there was already an infection before calculated and is still holding)
light infection (11-30°C), moderate infection (13-30°C), severe infection (16-30°C), array with leaf wetness duration (see above)
Reset: if rel. humiditiy is lower than 70%.
S. versicarium infections exists often together with Alternaria porri infections. Therefore the TomCast (Alternaria) model is quite frequent used for both.