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2001 Fruit and Vegetable Crops Research Report
Introduction ! Tree Fruits ! Small Fruits ! Vegetables ! Greenhouse Production ! Diagnostic Laboratory ! Appendix

Diagnostic Laboratory

Fruit and Vegetable Disease Observations from the Plant Disease Diagnostic Laboratory

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

Introduction

Diagnosis of plant diseases and providing recommendations for their control are the result of UK College of Agriculture research (Agricultural Experiment Station) and Cooperative Extension Service activities through the 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 percent 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 annual expenditure to support diagnosis of fruit and vegetable specimens by the laboratory is $15,000, excluding UK physical plant overhead costs.

Materials and Methods

Diagnosing fruit and vegetable diseases involves a great deal of research into the possible causes of the 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 assays (ELISA), electron microscopy, nematode extraction, or soil pH and soluble salts tests. Diagnoses that require consultation with UK faculty plant pathologists and horticulturists, and which need culturing and ELISA, are common for commercial fruits and vegetables. The laboratory also has a role in monitoring pathogen resistance to fungicides and bactericides. These exceptional measures are efforts well-spent because fruits and vegetables are high value crops for Kentucky. Computer-based laboratory records are maintained to provide information used for conducting plant disease surveys, identifying new disease outbreaks, and formulating educational programs.

Following a mild fall, temperatures in December 2000 were 10 degrees below normal, and tree fruits not hardened off showed winter injury symptoms during the 2001 growing season. Kentucky early spring temperatures, while lower than normal in March, were well above normal in February and April, leading to early and sustained bloom on many fruit crops. Hard freezes occurred during bloom on April 18 and 19 causing fruit injury and affecting some diseases. March and April were drier than normal (April received only 1.4 inches of rain).

Results and Discussion

The following are new and emerging fruit and vegetable diseases in Kentucky:

Tree Fruit Diseases

Dry weather in March and April reduced the occurrence of primary infections of apple scab (Venturia inaequalis). Nevertheless, there was just enough moisture to favor significant cedar rust (Gymnosporangium juniperi-virginianae, G. clavipes, and G. globosum) infections. Unusually warm April weather and occasional showers during apple and pear bloom resulted in devastating fire blight (Erwinia amylovora) outbreaks statewide. Spring frosts occurred and may have exacerbated fire blight and also caused apple fruits to show russeted equatorial bands later in the season. Seasonal summer rains (actually excess rain in July, with 7.3 inches) and long leaf wetness periods increased the incidence and severity of peach scab (Cladosporium carpophilum), secondary apple scab, apple frogeye leaf spot (Sphaeropsis malorum), apple sooty blotch (Peltaster fructicola, Geastrumia polystigmatis, Leptodontium elatius, and other fungi), and flyspeck (Zygophiala jamaicensis), all of which are enhanced by long leaf wetness periods. By season's end, susceptible unsprayed apples had less scab than usual, but fruits were covered with sooty blotch and flyspeck. Bitter rot (Colletotrichum gloeosporioides) was found in some apple orchards.

Small Fruit Diseases

Blackberries in some regions of Kentucky suffered winter injury. Systemic orange rust (Gymnoconia nitens) was devastating to blackberries in some locations. Blackberry rosette (Cercosporella rubi) was also observed. Tobacco ring spot virus (TRSV), causing mosaic symptoms and crumbly, unproductive berries, was found infecting blackberries in eastern and western Kentucky. A possible outbreak of impatiens necrotic spot (INSV) or another related virus is under investigation. Wet July weather and poorly drained soils stimulated root rot (Phytophthora spp.) of raspberries. Grape crown gall (Agrobacterium tumefaciens) incidence was up, and black rot (Guignardia bidwellii) and anthracnose (Elsinoe ampelina) were also prevalent. Pierce's disease (X. fastidiosa) was discovered for the first time in Western Kentucky (see separate report on this disease). This disease can be devastating to grape production—much more Kentucky research is needed. Strawberry anthracnose (Colletotrichum acutatum) and strawberry leaf spot (Mycosphaerella fragariae) occurred early in the season.

Vegetable Diseases

Due to a hot, wet summer in many areas of the state, infectious diseases significantly affected the success of production of commercial vegetable crops.

Vegetable Transplants. Several diseases were diagnosed from vegetable transplant production within the state. These included Tomato Mosaic Virus (ToMV), Tobacco Mosaic Virus (TMV) in tomatoes, Tomato Spotted Wilt Virus (TSWV), and Impatiens Necrotic Spot Virus (INSV) of tomato and pepper transplants. The first two viruses were probably from seed-borne sources, while the latter two likely came from other plants being grown in the same greenhouse. INSV may have developed as a result of vegetable transplants being produced in the same greenhouse with virus-susceptible ornamental plants such as petunia and impatiens.

Cole Crops. Diseases diagnosed included wirestem (Rhizoctonia solani) on transplants and newly set cole crops including cabbage, broccoli, and cauliflower. The fungus also caused stem and head rots later in the season. Blackleg of broccoli (Phoma lingam), leaf spot of cabbage and broccoli (Alternaria spp.), and cabbage yellows (Fusarium oxysporum) were found several times. Pythium root rot was diagnosed from several transplant operations involving the float system, as was a very serious spiral root disorder on cabbage seedlings. Bacterial diseases included soft rot of the heads (Erwinia and Pseudomonas) and black rot (Xanthomonas campestris). Turnip diseases included Cercosporella leaf spot and anthracnose. Boron deficiency was also common in several crops. One or more aphid-borne viruses were observed in several cases, especially in fall plantings.

Tomatoes. Commercial tomato plantings were infected by several bacterial diseases including bacterial canker (Clavibacter michiganensis), bacterial spot (Xanthomonas campestris pv. vesicatoria), bacterial speck (Pseudomonas syringae pv. tomato including some strains that are copper resistant), bacterial wilt (Ralstonia solanacearum), and pith necrosis (Pseudomonas corrugata). The copper-resistant speck is of particular concern.

With the protracted hot and wet season, a much higher disease potential was present from the two major fungal leaf spots, early blight (Alternaria solani) and Septoria leaf spot (Septoria lycopersici). These were controlled well with the strobilurin fungicides where good application methods were followed. Powdery mildew was present again in greenhouses, but much less was noted in the field this year. Fungal stem diseases that took their toll included Fusarium wilt (Fusarium oxysporum f.sp. lycopersici), Phytophthora stem canker (Phytophthora spp.), Botrytis stem canker (Botrytis cinerea), timber rot (Sclerotinia sclerotiorum), and southern stem blight (Sclerotium rolfsii). The latter was especially serious in some situations where post-plant herbicides were applied late.

A number of viral diseases were present and caused some major losses associated with TSWV and ToMV and/or TMV. More common viruses resulting in minor losses were Tobacco Etch Virus (TEV), Potato Virus Y (PVY), Alfalfa Mosaic Virus (AMV), and Cucumber Mosaic Virus (CMV). Root knot nematodes, both Meloidogyne incognita and M. hapla, caused losses in several plantings. Root and stem infections (Pythium spp. and Rhizoctonia spp.) were also present. Fruit diseases included all the fungal and bacterial leaf diseases above plus anthracnose and buckeye rot. Tomato fruit also experienced a number of the physiological disorders such as catfacing, blossom-end rot, growth cracks, blotchy ripening, yellow shoulders, and sunscald.

Peppers. Phytophthora blight (caused by Phytophthora capsici) is emerging as a major pepper disease in Kentucky, especially in wet sites following pumpkins or tobacco. Bacterial canker (Clavibacter michiganensis subsp. michiganensis) is another emerging major disease threat to peppers. This disease is probably coming in with the seed. Bacterial spot (Xanthomonas campestris pv. vesicatoria) remains an important problem but is declining with increased use of resistant varieties. Viruses are also increasing in importance, including several serious cases of TSWV, AMV, the potyvirus complex (mainly TEV), CMV, and TMV. Fruit anthracnose (Colletotrichum spp.) is increasing in importance, especially with the hot peppers. Occasionally southern stem blight (Sclerotium rolfsii) and stem rot (Rhizoctonia solani) were problems. Fusarium stem rot (starting in the greenhouse and continuing in the field), Rhizoctonia damping off, and Pythium root rot were often found. Pyllosticta leaf spot was found several times.

Cucurbits. Cucurbit crops are becoming more popular in Kentucky and their diseases are increasing in economic importance. Phytophthora root rot, stem rot, leaf blight, and fruit rot (Phytophthora capsici) are widespread in the state and cause great losses in many fields of pumpkins, squash, and cucumbers. Microdochium blight (Microdochium sp. recently renamed Plectosporium) was widespread and caused considerable damage in most fields that were not being sprayed well. This disease also developed strongly in some fields that had been sprayed regularly but where poor timing, poor coverage, or the wrong fungicides were involved. Like many other diseases, pumpkin fruit rot incidence is associated with a failure to use crop rotation away from other vegetables or tobacco. Fusarium (Fusarium spp.) fruit rots were a common problem again this year on pumpkin and winter squash along with Phytophthora blight. Nutritional disorders were also common, including several cases of manganese toxicity and blossom-end rot.

Anthracnose (Colletotrichum spp.), gummy stem blight/black rot (Mycosphaerella melonis), downy mildew (Pseudo-peronospora cubensis), and powdery mildew (Sphaerotheca fuliginea or Erysiphe cichoracearum) were found at serious levels in some fields on many of the cucurbits. Alternaria leaf blight of melons was much more active this year than normal, occurring earlier and causing more damage. The potyvirus complex, dominated by Watermelon Mosaic Virus (WMV), was widespread in pumpkin and winter squash, while several cases of CMV were also found in melon crops. Bacterial diseases of cucurbits were frequent and included angular leaf spot (Pseudomonas syringae pv. lachrymans), a bacterial fruit rot (Xanthomonas cucurbitae), and bacterial wilt (Erwinia tracheiphila). The latter has always been a major problem in cantaloupe and cucumber, but is now becoming more common in squash and pumpkin.

Symptoms of a newly emerging bacterial disease, Cucurbit Yellow Vine Decline (Serratia marsescens), were found in watermelon, muskmelon, summer squash, and winter squash. We have not yet proven that this pathogen was the causal agent; the disease was not identified until late summer (thanks to some excellent work by plant pathologists in Oklahoma). Based on symptoms present, it appears that this disease is now active in Kentucky as well as in other states to our west and southwest.

Other Vegetable Crops. Sweet corn rusts (Puccinia graminis and P. sorghi) were widespread again this year, with significant levels of Stewart's wilt (Pantoea [Erwinia] stewartii subsp. stewartii), and maize dwarf mosaic virus (MDMV) being observed. There were also isolated cases of anthracnose (Colletotrichum graminicola). Asparagus crown rot (Fusarium sp.), bean root and stem rot (Pythium spp., Rhizoctonia solani and Fusarium solani f.sp. phaseoli), bean anthracnose (Colletotrichum lindemuthianum), bean rust (Uromyces appendiculatus), bean common bacterial blight (Xanthomonas campestris pv. phaseoli), bean virus complex (mainly bean yellow mosaic virus), potato scab (Streptomyces scabies), and sweet potato scurf (Monilochaetes infuscans) were frequently observed this year. A severe case of soil pox of sweet potatoes (Streptomyces ipomoea) was associated with a high soil pH situation, a reminder that cultural practices markedly affect disease development. Okra diseases included Rhizoctonia root and stem rot, black root rot, powdery mildew, and root knot nematodes.

The laboratory has been conducting a survey of viruses infecting commercial vegetables in Kentucky for the past several years. Using ELISA tests, a broad range of virus diseases was found; no new viruses were detected in 2001. Growers are urged to bring to the attention of their County Extension Agent any observations of new outbreaks and disease trends in their fields. We want to be especially watchful of the new spectrum of microbes and diseases that may occur with changes in fungicide use patterns from broad-spectrum protectant fungicides such as mancozeb and chlorothalonil to new chemicals such as Quadris and Abound. These latter products present greater risks of pathogen resistance to the fungicide while incurring reduced risks to human health and the environment. For example, we have noted increased bacterial diseases in tomatoes and now want to know if this is related to how we raise our crops or manage other diseases or to sources of seeds and transplants.

Because fruits and vegetables are high value crops, the Plant Disease Diagnostic Laboratory should be of great value to commercial growers. Many growers, however, are not using the laboratory often enough or they are waiting until their disease problem has become well established. By then, it may be too late to do anything about it or, in some cases, to correctly diagnose the sequence of diseases that may have led to the final outcome. Growers need to consult on a regular basis with their County Extension Agents so that appropriate plant specimens are sent to the laboratory in a timely manner. We are urging County Extension Agents to stress the need for accurate diagnosis of diseases of high value crops. Growers can work with their agents to ensure that they have the best possible information on fruit and vegetable diseases.

Literature Cited

  1. Bachi, P.R., J.W. Beale, J.R. Hartman, D.E. Hershman, W.C. Nesmith, and P.C. Vincelli. 2002. Plant Diseases in Kentucky—Plant Disease Diagnostic Laboratory Summary, 2001. UK Department of Plant Pathology. In press.

Introduction ! Tree Fruits ! Small Fruits ! Vegetables ! Greenhouse Production ! Diagnostic Laboratory ! Appendix


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