The weather has been mostly cloudy and very humid for most of the past week. Combined with moisture stress and previous or continued damage from Alternaria, this has led to the development of Botrytis blight. The humid conditions have allowed the lesions of Botrytis blight to spread on leaves in such a way as to develop an Alternaria-like appearance. These two diseases can be easily confused under these conditions. Concentric rings are appearing in the Botrytis lesions (Figure 1), which are usually typical of Alternaria leaf blight. However, Botrytis lesions lack the typical yellow halo that is characteristic of Alternaria (Figure 2), and that is the best way to distinguish them. If Alternaria has been killed by a fungicide it will lose the yellow halo, but the lesions will become dry and often break apart and often partially fall off the plant.
There are several good fungicide options to control Botrytis blight. These include Elevate, Bravo/Echo, Captan/Maestro, Fontelis, Scala, Scholar and Switch. A lack of control of Botrytis may be due to the use of too many Alternaria-specific products consecutively. Part of this may be due to a chlorothalonil (Bravo/Echo) shortage and too much reliance on mancozeb products which have minimal effect on Botrytis. The current weather is still very conducive to the development of Alternaria leaf blight, and products should be chosen that have good control of both diseases.