Last fall, my lab received a carrot sample with disease-like lesions (Figures 1 and 2). There are at least 3 carrot diseases that may appear similar. These diseases are: Alternaria leaf blight (late blight), Cercospora leaf spot (early blight) and bacterial leaf blight. Often an examination in the laboratory is necessary. My examination revealed the characteristic spores (conidia) of Alternaria dauci, causal agent of Alternaria leaf blight.
Figure 1 shows a stand of carrots with several leaves that appear chlorotic (yellow) and necrotic. A closer examination reveals small lesions on the leaves (Figure 2). Loss of leaves may lead to fewer or smaller carrots. Sometimes severe infections can lead to the premature separating of the leaves and root.
Figure 1. A stand of carrots with chlorotic leaves due to Alternaria leaf blight of carrot (Photo by Wenjing Guan).
Figure 2. Necrotic lesions caused by Alternaria dauci on carrot leaves. (Photo by Dan Egel)
Alternaria leaf blight can be rapidly spread between plants by the conidia that are produced on the plant surface. I could easily find these spores on the surface of the carrot leaves brought to my lab. The conidia may germinate to cause infection with as little as 2 hours of leaf wetness. It is easy to imagine spread of the disease with wind and rain.
Since leaf wetness is necessary for disease initiation and spread, avoid overhead irrigation. The disease may be spread on seeds, therefore care should be taken when purchasing or saving seed. Carrot varieties may vary in susceptibility; ask your seed representative for varieties with partial resistance. Crop rotations of at least two years will help to lessen disease severity. When carrot production is complete, old crop residue should be plowed under as soon as possible.
Finally, several fungicides may help to manage this disease. With the addition of Michigan State University to your Midwest Vegetable Production Guide (ID-56) team, some modifications have been made. For example, application of fungicides for Alternaria leaf blight of carrot may be scheduled with TOM-CAST (use 15 disease severity values). See details in your 2017 ID-56 or mwveguide.org for more details. Organic gardeners may find copper hydroxide or copper sulfate formulations that are approved for use.