This disease was recently observed in southern Indiana. Moderate damage to leaves in broccoli does not usually cause yield or quality loss. However, Alternaria leaf spot may also affect Chinese cabbage, bok choy and leafy brassicas such as cabbage.
The most common symptom is a leaf spot (Figure 1). The lesions start out small; as lesions become larger they may have a light brown border with a light gray center. Lesions may also occur on leaf petioles. Under conducive conditions, some defoliation may occur, affecting the size or quality of broccoli heads. In severe cases, broccoli heads may be affected by this disease (See article in this issue about heat damage to broccoli heads).
Figure 1. Alternaria leaf spot of broccoli may cause a round lesion with a light brown border and a gray center.
There are two species of the fungus that may cause Alternaria leaf spot. The species that was isolated recently (Alternaria brassicicola) is favored by temperatures from 68 to 86°F and 12 hours of 90% relative humidity. In southern Indiana, we have had a relatively warm and dry late summer and fall. However, dew periods in combination with overhead irrigations and light rains may be conducive to this disease.
The fungus that causes Alternaria leaf spot of broccoli may survive in crop residue. Therefore, crop rotation and fall tillage will help to lessen the severity of this disease. No-tillage systems may increase the severity of Alternaria leaf spot of broccoli. Some cultivars are more susceptible to this disease than others; pay close attention to differences in disease severity among cultivars and discuss this disease with your seed representative. In addition, this disease may be transmitted in seed.
Foliar fungicides my help to reduce the severity of Alternaria leaf spot although light infections may not require fungicides. The Midwest Vegetable Production Guide for Commercial Growers lists several recommended products. These include: products with the active ingredient azoxystropin (e.g., Quadris®, Satori ®and Quadris Top® with difenconazole); Cabrio EG; several products with chlorothalonil as an active ingredient (e.g., Bravo®, Echo®, Equus®, Initiate®); Endura®; Fontelis®; Inspire Super®; Reason 500SC®; Switch 62.5 WG®.