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PR-410: 1998 Fruit and Vegetable Crops Research Report

Diagnostic Laboratory

IntroductionTree FruitsSmall FruitsVegetablesDiagnostic LaboratoryAppendix A

Fruit and Vegetable Disease Observations from the Plant Disease Diagnostic Laboratory

Julie Beale, Paul Bachi, William Nesmith, and John Hartman, Department of Plant Pathology


Plant disease diagnosis is an ongoing educational and research activity of the UK Department of Plant Pathology. We maintain two branches of the Plant Disease Diagnostic Laboratory: one on the UK campus in Lexington and one at the UK Research and Education Center in Princeton. Of the more than 4,000 plant specimens examined annually, approximately 5% are commercial fruit and vegetable plant specimens (1). Although there is no charge to the growers for plant disease diagnosis at UK, the estimated direct expenditure to support diagnosis of fruit and vegetable specimens by the laboratory is $8,500, excluding UK physical plant overhead costs.

Materials and Methods

Making a diagnosis involves a great deal of research into the possible causes of the plant problem. Most visual diagnoses include microscopy to determine what plant parts are affected and to identify the microbe involved. In addition, many specimens require special tests such as moist-chamber incubation, culturing, enzyme-linked immunosorbent assay (ELISA), electron microscopy, nematode extraction, or soil pH and soluble salts tests. Diagnoses that require consultation with UK faculty plant pathologists and horticulturists, and that need culturing and ELISA, are common for commercial fruits and vegetables. These exceptional measures are efforts well spent because of the high value of fruits and vegetable crops in Kentucky. Computer-based laboratory records are maintained to provide information used for conducting plant disease surveys, identifying new disease outbreaks, and formulating educational programs.

Much of the 1998 growing season was very favorable for fruit and vegetable diseases. Kentucky orchards, vineyards, and farms experienced fluctuating, but mild, winter temperatures, an early-spring freeze, heavy late-spring and early-summer rains, and dry late-summer and fall weather, each of which contributed to the development of different kinds of diseases.

Results and Discussion

Tree fruit diseases

An abiotic weather-related problem of tree fruits in some locations was the sudden collapse of shoots and foliage at the first onset of warm spring weather. This dieback could be attributed to temperatures below 10°F on March 12, at which time most plants had broken dormancy; many peaches were in full bloom. Browning of phloem tissues was observed in cold-injured trees and followed by a variety of canker diseases. Mild winter temperatures and wet spring weather resulted in abundant peach leaf curl (Taphrina deformans). Peaches later showed considerable brown rot (Monilinia fructicola) fruit decay. Rainy spring weather favored apple scab (Venturia inaequalis) and cedar-apple rust (Gymnosporangium juniperi-virginianae). Rust-susceptible apples showed significant leaf spotting, while unsprayed apples were practically defoliated by August. In many apple orchards, white rot (Botryosphaeria dothidia) was the major fruit rot in late summer. Different types of leaf wetness monitors, used for plant disease management, were compared for accuracy and precision at the UK Experiment Farm orchard in western Kentucky.

Small fruit diseases

Strawberry leaf spot (Mycosphaerella fragariae) and strawberry scorch (Diplocarpon earlianum), affecting stolons and petioles, were apparently favored by wet spring and early summer weather. Similarly, wet weather and poorly drained soils stimulated root rot (Phytophthora spp.) of raspberries.

Vegetable diseases

Black rot (Xanthomonas campestris pv. campestris) was observed from commercial cabbage fields in the spring. Tomatoes in commercial plantings were infected by several different bacterial diseases this year. Bacterial spot (Xanthomonas campestris pv. vesicatoria) and bacterial speck (Pseudomonas syringae pv. tomato) were found in transplants and in the field. Bacterial canker (Clavibacter michiganensis) and pith necrosis (Pseudomonas corrugata) caused serious tomato losses. Powdery mildew (Leveillula taurica), a disease only recently found in the United States, was observed on Kentucky greenhouse tomatoes.

Peppers continue to develop bacterial leaf spot (Xanthomonas campestris pv. vesicatoria), and the plant disease diagnostic laboratory is monitoring for the possible breakdown of the single-gene resistance to races 1, 2, and 3 of the bacteria in cultivars like ‘Boynton Bell’, ‘Enterprise’, etc. The laboratory has also uncovered a root and stem rot complex of pepper involving the fungus Fusarium; it is possibly a new disease.

Pumpkins and other cucurbits are becoming more popular in Kentucky, and their diseases continue to be economically important. Powdery mildew, caused by two different fungi (Sphaerotheca fuliginea or Erysiphe cichoracearum), is serious every year, so the laboratory is monitoring for development of powdery mildew strains that may be resistant to currently available fungicides. Downy mildew (Pseudoperonospora cubensis) was present at high levels in some fields in the fall.

The laboratory has been conducting a survey of the viruses infecting commercial vegetables in Kentucky for the past three years. Using ELISA tests, a broad range of virus diseases were found; no new viruses were detected in 1998.

The plant disease diagnostic laboratory has been shown to be of great value to some fruit and vegetable growers. However, many commercial growers are still not using the plant disease diagnostic laboratory often enough, or they are waiting until their disease problem has become well established. By that time it may be too late to do anything to treat the problem or in some cases to correctly diagnose the sequence of events/diseases that may have led to the final outcome. Growers are urged to consult with their county Extension agents on a regular basis so that appropriate plant specimens are sent to the laboratory in a timely manner.

Literature Cited

  1. Bachi, P.R., J.W. Beale, J.R. Hartman, D.E. Hershman, W.C. Nesmith, and P.C. Vincelli. 1999. Plant Diseases in Kentucky - Plant Disease Diagnostic Laboratory Summary, 1998. UK Department of Plant Pathology (in press).

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