ALTERNARIA DISEASES
Fungi in the genus Alternaria are responsible for several diseases of economic importance to Central Coast growers. Alternaria solani causes early blight of eggplant, potatoes, and tomatoes. Alternaria brassicae and A. brassicicola cause a leafspot and flower or bud rot on broccoli. Alternaria dauci causes a leaf blight of carrots, and Alternaria radicina causes a crown blight in carrots. All these fungi can cause severe plant damage at any time of year if temperature and moisture conditions are favorable. On plants with a broadleaf Alternaria infections usually are expressed as a circular gray, brown lesion with a yellow margin (halo). Many of these lesions will show concentric dark rings similar in appearance to annual growth rings in wood. The circular lesion with the concentric rings is very diagnostic for Alternaria. Few other vegetable pathogens show this symptom. The yellow halo that surrounds Alternaria lesions is the result of a toxin produced by the fungus.
Two species of Alternaria attack broccoli. Chinese cabbage, pok choy, cabbage, collards, turnips, radish, and other crucifers can also be hosts for these fungi. In Chinese cabbage, pok choy, and cabbage the leafspot can be severe during the winter and spring months. In broccoli, the flower or bud rot causes damage almost every fall and winter season on the Central Coast. Initially, individual flower buds are attacked. The fungus spreads quickly to surrounding flower buds (beads) and a gray to black, fuzzy spot is soon evident. One strike on a broccoli head is sufficient to render the head unmarketable. On the leafy crucifers, heads often have to be stripped down to remove spotted leaves. This reduces weight and may make the head too small to market.
The Alternaria spp. in carrot both cause blight diseases. Affected foliage first develops small dark-brown to black spots with a yellow border. Affected leaves die rapidly, turning gray-brown, often with a black fuzz present, which is the fungus sporulating on the tissue. Decreased leaf surface causes poor growth and small carrots. Lack of top foliage will reduce harvesting efficiency and may prevent the carrot from being pulled during the harvesting process. The crown blight of carrots attacks the host at the root-shot interface. A black rot develops, and the tops wilt and topple over. Research indicates that A. radicina, the crown blight pathogen, is seedborne, and clean seed and/or seed treatment are effective ways to combat this disease.
The Alternaria spp. that attack foliage and flower parts are best controlled by fungicides. Research has shown that chlorothalonil, maneb, iprodione, mancozeb, and fentin are effective against Alternaria fungi. Applications need to begin at first sign of disease, and preventive sprays may be necessary when weather conditions are optimum for disease development. The cardinal temperatures for growth of A. brassicola and A. brassicicola are 46 (low), 75 to 82 (optimum range) and 97°F (high). Rain or dew that persists for more than 9 hours is essential for infection to occur.
Check fungicide labels for crop registration and use rates.