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(Reprinted with permission - May '99 issue of Citrus & Vegetable Magazine) Watermelons are a major crop in most production regions of Florida. Diseases such as gummy stem blight and Fusarium wilt are perennial problems for commercial watermelon growers and can cause significant plant losses every year. This year in Southwest Florida, both diseases were prevalent and losses of 20 percent and higher occurred in at least three fields due to Fusarium wilt. Other diseases such as downy mildew and Phytophthora blight were observed sporadically this spring at low levels. Field diagnosis of disease problems is usually based upon plant symptoms and signs of the pathogen. Identification of any disease by symptoms alone is at best tentative because there are many different agents (biological and physical) that may cause similar symptoms or plant reactions. However, for some problems, a tentative field diagnosis may be made by "diagnostic symptoms" of the unique symptoms that are typical for a particular disease and/or by identification of fungal structure (signs) on or in plant tissue. Tentative field identification of diseases may help to catch disease problems early and shorten the time for a control response. Identification of the pathogen by fungal structures such as fruiting bodies or spores on a symptomatic plant can accurately identify many fungal diseases. The presence of a pathogen alone with typical symptoms should give an accurate diagnosis of the problem. There are some fungal structures that are visible with a 10x or higher hand lens. For example, when using a hand lens, the black, rounded pycnidia formed by the gummy stem blight fungus usually can be seen imbedded in stem tissue. Positive identification with a compound microscope at 100x or higher is necessary to identify some fungal signs, such as the spores of Fusarium or Phytophthora sp. The high magnification is necessary to identify the fungus characteristics and in addition, it may be necessary to first culture the fungus on artificial media to confirm its presence. The following is intended to briefly describe some common disease problems of watermelon and to give some guidance in making tentative field identification of these diseases. Identification of the pathogen by a trained specialist using appropriate diagnostic tools is recommended for confirmation.
Fusarium wilt is caused by the fungus, Fusarium oxysporum f. sp. niveum, and is a major problem in watermelon production throughout Florida in susceptible varieties. While this disease is capable of causing damping off of young seedlings, the most visible symptom in the field is the severe wilt of one or more runners that may lead to plant collapse. Visual identification of Fusarium wilt is done by examining plants that are wilting but still green. There may be slight stem gumming and an elongated lesion on one side of stem that is dark brown. No fungal signs can be detected without the aid of a microscope. The most consistent and reliable diagnostic symptom of Fusarium wilt is vascular discoloration that is observed when the stem or roots are cut in longitudinal or cross sections. The vascular tissue appears as a brown circle in the interior of the stem and the surrounding stem tissue remains green when the root or lower stem is cut crosswise. If the root or stem is cut in half longitudinally, two parallel bands of discolored tissue will be seen in infected tissue. The discoloration may be dark-yellow, brown or red.
Gummy stem blight is caused by the fungus Didymella bryoniae. This disease can be very severe on young seedlings before or after transplanting and causes rapid stem girdling and death. Infections in the stems of plants may cause wilting. It also causes brown spots on the leaves. On the stem, regardless of the age of the lesion, identification of the fungal fruiting bodies (perithecia or pycnidia) on the greenish stem lesions confirms gummy stem blight. The structures are usually roundish, shiny black, appear as black specks and usually may be seen with a hand lens. On older lesions, profuse stem gumming may be apparent and appears as amber to reddish-orange, gummy exudate.
Phytophthora blight is caused by the fungus, Phytophthora capsici. Previously, only the fruits of watermelon were considered susceptible to this disease. Severe foliar and crown rot epidemics occurred last year in fields in Southwest and Central Florida. Symptoms in watermelon fruit appear as tannish-brown to black, watersoaked lesions that may have a white covering of fungal mycelial and sporangia growth. The foliar phase of Phytophthora blight appears initially as brownish-tan lesions on the leaves Identification of this disease for the foliar phase requires a compound microscope to distinguish fungal sporangia and mycelium on the leaf surface. The crown rot phase is characterized by rapid plant wilt and death within a few days. Symptoms on the stems and roots appear as light to dark brown to black lesion and the tissue becomes soft and watersoaked. The lesion extends throughout the stem or crown tissue and is not confined to the vascular system like Fusarium wilt. Downy mildew is strictly a foliar disease that is caused by the fungus Pseudoperonospora cubensis. Initial symptoms are yellow leaf spots that may or may not be angular due to delineation by small leaf veins. Spots will eventually turn brown to black in color and the leaf often curls and cups inward toward the midrib. The diagnostic characteristic for this disease symptom is the covering of downy, whitish-gray to light-blue fungal growth that occurs on the lower leaf surface in the individual spots. This downy growth is easily seen with a hand lens or the unaided eye. Lesions of downy mildew can appear similar to those of gummy stem blight and anthracnose. Thus, the identification of fungal structure is critical. More than one disease can occur on the same leaf at the same time. Two other leaf spots that occur sporadically in Florida are Alternaria leaf blight and Anthracnose. Alternaria leaf bight (Alternaria cucumerina) causes leaf spots that are initially small and yellow, irregular in shape, and eventually turn brown. The spots may expand and form concentric zones Identification of Alternaria, leaf spot is determined by the presence of the distinct spores of Alternaria when viewed under a microscope. Anthracnose, caused by the fungus Colletotrichum orbiculare, causes angular- or irregular shaped, brown-to- black leaf spots usually surrounded by a yellow box den Lesions of anthracnose also occur in the stems and fruit. A small, black speck that is a fruiting structure, or masses of pink spores, may be present on fruit and leaf lesions. Fruit lesions may be raised, pimple-like bumps initially, but later the bumps collapse, and the spot appears as a sunken lesion. CVM P.D. Roberts, assistant professor, Plant Pathology, UF's Southwest Florida Research and Education Center, Immokalee 941-658-3400; T.A. Kucharek professor, Plant Pathology, University of Florida, Gainesville 352392-1980 and G. McAvoy, vegetable extension agent, Hendry County, 941-674-4092. |
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