P&PDL Picture of the Week for
July 12, 2010

Tomato Blights and Wet Weather

Gail Ruhl, Senior Plant Disease Diagnostician, Purdue University

Two major fungal blights that regularly wreak havoc on tomatoes in Indiana are Septoria leaf spot and early blight, caused by the fungus Alternaria solani. Both of these fungal diseases are present this year throughout the state. A third disease, Late Blight,caused by the fungal-like organism, Phytophthora infestans, made the news last year for devastating tomato crops and has been confirmed in one county in Southern Indiana this year. These three tomato diseases are spread by spores that require dew or rain to infect the plant and thus they are most severe under wet conditions.

Septoria leaf spot is caused by the fungus Septoria lycopersici and usually appears on the lower leaves after the first fruits set. Initially the fungus causes numerous, small, roughly circular spots, scattered randomly over the leaf. Spots enlarge to a size of approximately 1/16 to 1/4 inch in diameter with dark brown borders and tan or light colored centers. Septoria leaf spot is sometimes confused with bacterial spot of tomato. The presence of fruiting bodies of the fungus, visible as tiny black specks in the centers of the spots, confirms Septoria leaf spot. The fungus is spread by splashing water and by working among the plants when they are wet. Heavily infected leaves will turn yellow, dry up, and drop off. Fruits are rarely infected however, defoliation due to infection by Septoria leaf spot may result in sunscalding of the fruit. This fungal disease overwinters on diseased leaf material.

Early blight, caused by the fungus Alternaria solani, also appears on the lower leaves, usually after fruit set. The spots are dark brown to black and distinguished from Septoria by their larger size and concentric rings that develop in the spot forming a bull’s eye. The leaf area around each target spot turns yellow, and soon the entire leaf turns yellow and drops. Early blight fungus also infects stems and may produce stem cankers. It occasionally attacks the fruit, producing large sunken black target spots on the stem end of the fruit. Infected fruits often drop before they mature. This disease is most common late in the growing season. The fungus overwinters on old tomato vines.

Late blight, (BP-80-W pdf file) caused by the fungal-like organism Phytophthora infestans, occurs in moist weather with cool nights and moderately warm days. Dark-green to nearly black wet-looking areas develop on leaf margins, spreading in from the leaf edge. In wet weather, the spots produce a downy, white growth of mycelia and spores on the lower leaf surface. Fruits also become blighted. Late blight is usually seen first on shoulders of tomato fruit as gray-green and water-soaked lesions that enlarge and turn dark brown and firm, with a rough surface. When conditions are favorable, the disease may progress very rapidly. Refer to the following webpage for additional information on late blight.

Management of these tomato blights includes both cultural as well as chemical measures. One can help reduce the severity of these diseases by using healthy transplants and rotating crop placement in the garden. Plant tomatoes in the same place only once every two or three years. Space plants and use cages to provide maximum air circulation in order to reduce leaf wetness.. Remove infected lower leaves and discard to help reduce the amount of disease present. Water at the base of the plants to avoid splashing of water that will spread spores. Avoid watering with overhead sprinklers in late afternoon or evening to prevent excessive moisture retention on foliage. If the plants stay wet all night, leaf spot infections are more likely to occur. Remove and destroy infected tomato vines in the fall. Plow or till to bury the remaining crop refuse. Clean up tomato cages in the fall before storing for the winter to avoid reintroducing infected plant tissue into the garden next spring.

Use fungicides when needed. These diseases spread rapidly and are difficult to control once established. Fungicides must be applied BEFORE disease becomes widespread on leaves and reapplied throughout the growing season. Retail products containing the active ingredient chlorothalonil may help reduce the spread of these foliar blights if applied on a regular basis. Be sure to follow all label directions.

Additional resource:
http://www.extension.iastate.ed
u/Publications/PM1266.pdf

Click image to enlarge

Septoria blight

Septoria leaf spot
Photo by Paul Bachi, University of KY

Septoria blight

Septoria leaf spot
Photo by PPDL

Early blight

Early blight
Photo by Paul Bachi, University of KY

Early blight

Target-shaped early blight lesion on stem
Photo by Dan Egel

Late blight

Late blight on leaf
Photo by Dan Egel

 

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service