Disease Management

Tomato Early Blight Leaf

Early Blight of Tomato and Potato

Early blight caused by Alternaria solani occurs wherever potatoes and tomatoes are grown. Uncontrolled, the disease may cause serious defoliation, resulting in decreased yield and quality.

A .solani survives between crops on infected plant debris, soil, other solanaceous host weeds and can be carried on tomato seed and infected tubers. The fungus enters the leaves directly or through wounds. Primary infection can occur on older foliage early in the season, but most secondary spread occurs as the plants age. Actively growing, young tissue and vigorous plants with adequate nitrogen generally do not express symptoms. Infection is favored by mild, rainy weather.

Early blight occurs on the foliage, stem, and fruit of tomato. It first appears as small brown to black lesions on older foliage. The tissue surrounding the initial lesion may become yellow, and when lesions are numerous entire leaves may become chlorotic. As the lesions enlarge, they often develop concentric rings giving them a ‘bull’s eye’ or ‘target-spot’ appearance. In the late summer when conditions are favorable for disease development, lesions can become numerous and plants defoliated, reducing both fruit quantity and quality. Fruit can become infected either in the green or ripe stage through the stem attachment. Lesions can become quite large, involve the whole fruit, and have characteristic concentric rings. Infected fruit often drop and losses of 30-50% of immature fruit may occur. Foliar symptoms on potato are quite similar, though defoliation rarely results. Tuber lesions are dark, sunken, and circular often bordered by a purple to gray raised tissue. The underlying flesh is dry, leathery, and brown. Lesions can increase in size during storage and tubers become shriveled.

Management

• Use resistant or tolerant cultivars.
• Start with disease-free seed, transplants, and seed tubers.
• Use long crop rotations, eradicate weeds, and eliminate volunteer plants and cull piles.
•  Plow under plant debris after harvest.
• Fertilize properly and keep plants growing vigorously.
• Handle seed tubers carefully to prevent wounding.
• Permit tubers to mature in the ground before harvesting and avoid bruising when handling.
• Spray regularly with fungicides. Spray applications should be scheduled by spore trapping or forecasting systems (TOM-CAST) to be most effective. Early season applications often fail to control secondary spread of the disease.

Chemical recommendations:

azoxystrobin (Quadris): 5 to 6.2 fl oz/A (0 dh, REI 4 h). Do not apply more than two sequential applications before alternating with a fungicide with a different mode of action.

chlorothalonil (Bravo Ultrex 82 WDG): 1.3 to 1.8 lb/A (0 dh, REI 12 h). Good rotation partner for Quadris.

Dimethomorph plus mancozeb (Acrobat MZ): 2.25 lb/A (14 dh, REI 24 h). Do not make more than five applications per season.

Maneb/mancozeb (Maneb, Penncozeb, Manzate, Dithane): Rates vary according to formulation. See labels for rates and timing. (3 dh, REI 24 h).

pyraclostrobin (Cabrio EG): 8 to 12 oz/A (0 dh, REI 12h). Apply at the first sign of disease and repeat at 7-14 day intervals. Do not apply more than 6 applications per season or rotate with Quadris.

Prepared by M. Bess Dicklow, UMass Extension, 107 Fernald Hall, University of Massachusetts, Amherst, MA 01003-9320. Tel.413-577-1827, Fax 413-545-2115. mbdicklo@umext.umass.edu