Early blight and its look-alike cousin, brown leaf spot, are causing confusion among some potato growers in Michigan.
Proper identification is crucial because some products used to control early blight are ineffective against brown leaf spot, according to a Michigan State University grower newsletter.
Early blight is caused by Alternaria solani, whereas brown leaf spot is caused by Alternaria alternata.
Both organisms overwinter as spores and mycelia on infected tissue of solanaceous plants.
The solanaceous family includes tomatoes, potatoes, peppers, eggplants and nightshade.
Warmer weather prompts fruiting and the release of spores, which are moved by wind and water onto potato plants.
On infected leaves, early blight produces lesions with dark concentric rings.
Brown leaf spot, on the other hand, causes numerous small lesions on leaves.
They'll grow and coalesce across large veins, turning the entire leaf brown.
Cultural controls, including crop rotation, planting certified seed, timely irrigation and post-harvest field sanitation, as well as fungicides help control brown leaf spot.
Strobilurins used for early blight will not control brown leaf spot adequately.
But protectant fungicides, such as maneb, mancozeb and thlorothalonil, used against early blight also are moderately effective against brown leaf spot when applied at seven to 10-day intervals.
Download the new MSU publication, "Michigan Potato Diseases: Brown Leaf Spot," for free at http://web2.msue.msu.edu.