Departments of Plant Pathology and Entomology
North Carolina Cooperative Extension Service
North Carolina State University

FRUIT
AND
VEGETABLES

North Carolina Pest News
Volume 11, Number 4, May 10, 1996
Stephen J. Toth, Jr. and Thomas A. Melton, editors

The information and recommendations in this newsletter are applicable to North Carolina
and may not apply in other areas.

From: James F. Walgenbach, Extension Entomologist

Insect Activity on Apples

First generation codling moth populations are quite high in many orchards this season. Growers should be reminded that for this first generation of adults, insecticide sprays are not needed at the first sign of high trap catches. There is a significant lag between emergence of adults and the initiation of egg laying. The most accurate method for timing a spray against this first generation is to apply the spray 250 degree days (50 base) after the first moth is caught in a trap (= biofix). In orchards with high trap catches (peak catch exceeds 50 moths per trap per week), a second application should be applied 14 days later. Although the biofix date varies depending on location, biofix date in Wilkes County was April 23, and April 25 in Henderson County. At the Mountain Horticultural Crops Research Station (Fletcher, North Carolina), we have accumulated about 150 degree days since biofix, and I expect to reach 250 degree days May 15 or 16. Finally, if trap catches do not exceed 10 per trap per week during the first generation, it is doubtful that a spray is needed. Insecticides of choice at this time are Guthion or Imidan.

From: Frank Louws and Paul Shoemaker, Extension Plant Pathologists

Tomatoes: Foliar Diseases in North Carolina

The most common foliar diseases encountered in North Carolina are three fungal diseases (early blight, Septoria leaf spot and late blight) and three bacterial diseases (bacterial canker, bacterial spot and bacterial speck). The relative importance of these diseases varies across the state because of differences in climatic conditions between the Coastal, Piedmont and Mountain regions. Each disease is described below, followed by general recommendations for management of tomato foliar diseases in North Carolina.

Disease Descriptions:

Early blight is a common problem throughout the state. The disease can occur on any above ground tissue but usually begins as small brownish-black lesions on the older leaves. The spots enlarge rapidly and form concentric rings that give a diagnostic target spot appearance. Spores are wind-borne or splash-dispersed and the disease is favored by mild temperatures (75 degrees F plus or minus 7 degrees) and leaf wetness. Early blight progress is favored by low nitrogen, early host maturity, nematode populations, or other forms of stress such as soil moisture stress.

Septoria leaf spot occurs with moderate frequency in the Coastal Plain and Piedmont and very rarely in the mountains. Lesions are generally restricted to the foliage and symptoms often appear first on the lower leaves when the fruit first set. Lesions are small (about 1/8 to 1/4 inch), circular with dark brown margins with tan to gray centers that contain diagnostic black dots ("pycnidia") when viewed with a hand-held lens. The disease can be "explosive" during optimum conditions such as long periods of leaf wetness (14 hours) with a lot of splashing rain and mild temperatures between 68 and 77 degrees F.

Late blight occurs with minor frequency in the Coastal Plain and Piedmont but with moderate frequency in the mountains, in which case the effects can be very serious. Foliar symptoms include water-soaked leaf spots 1 inch or larger in diameter. The spots have a light green margin visible on the upper leaf surface and profuse downy mildew on the lower leaf surface. Stems and petioles may also become infected, turning black with the distal portions soon blighting. In many cases, the fungus is resistant to Ridomil and alternative fungicides need to be used, as outlined below.

Bacterial canker occurs with moderate frequency in the Piedmont and Mountain Region and rather infrequently in the Coastal Plain. However, when it does occur problems can be dramatic especially if the systemic phase of the disease occurs. In these cases, plants will initially show downward turning of lower leaves, usually on one side, followed by complete wilting and collapse of the plant. Cutting stems at the base often reveals a creamy to reddish discoloration, especially along the vascular tissue. The foliar phase is characterized by brown to black leaf margins and small tan spots on leaves and petioles. Fruit spots may develop raised brown centers surrounded by white to yellow halos that look like bird's eyes. The disease is seed-borne or may overwinter in infected tomato debris from the previous year.

Bacterial spot occurs with moderate frequency in the Coastal and Piedmont areas and is less common in the Mountain region. The disease is favored by wet warm weather and can be very destructive. All parts of the plant above ground can be affected, and lesions are generally brown and circular. Lesions can be confused with early blight, but do not form concentric rings and generally do not exceed 1/8 inch in diameter. Lesions may coalesce to form long streaks and defoliation can result. On fruit, lesions up to 1/4 inch in diameter have a raised scabby appearance. This disease is also seed-borne.

Bacterial speck prefers cool wet weather and therefore can be more of a problem in the Mountains than other regions of North Carolina. However, the disease generally occurs with minor frequency. Lesions on leaves initially are small, black and often surrounded by a yellow halo. All tissue above 2ground can be affected, and on fruit lesions appear as superficial black specks less than 1/8 inch in diameter. This bacteria can also be seed-borne.

Management Practices:

1. Short-term considerations:

Tomato transplants have been field set or will be planted soon, so what are the short-term options? The most effective method for managing the above diseases is through the proper selection and use of fungicides. Both mancozeb (Dithane, Manzate and Penncozeb, etc.) and chlorothalonil (e.g., Bravo) provide broad-spectrum control of fungal diseases. Copper-based fungicides (e.g., Kocide, Basic Copper Sulphate) are effective against the bacterial diseases when used in a preventive manner. In general, growers should use a preventive spray schedule combining either mancozeb or chlorothalonil with a copper-based material. Spray applications should begin immediately after transplanting. Because of differences across the state, application frequency varies. Sprays should be applied every 7 to 10 days in the Coastal region, every 7 days in the Piedmont, and every 5 days in the Mountains. The copper sprays may be omitted two weeks before harvest.

2. Longer-term and cultural considerations:

Early blight and Septoria leaf spot share a number of common features and can persist from one year to the next in association with tomato debris or wild relatives of tomato such as horse nettle. Infected transplants or contaminated seed, however, cannot be discounted as a source of inoculum. Based on this information, long-term management is facilitated by a three year crop rotation and ensuring weed hosts are eradicated. Late blight generally migrates from outside the field or region. Therefore, for tomatoes, there are few long-term strategies. In the case of bacteria, growers should use disease-free seed and allow up to three years between tomato crops. Irrigation schedules should ensure that the foliage is not wet as evening approaches and workers should remain out of the field until the foliage is dry.

Web page last updated on 13 May 1996 by Stephen J. Toth, Jr..