|Crop Knowledge Master||Fungi|
|gummy stem blight, black rot, canker (Plant Disease Pathogen)|
Stephen A. Ferreira, Extension Plant Pathologist
Rebecca A. Boley, Educational Specialist
Department of Plant Pathology,CTAHR
University of Hawaii at Manoa
Didymella bryoniae occurs on Cucurbits exclusively. Hosts include watermelon, melon, pumpkin, cucumber, squash, bittermelon, smooth luffa, and angled luffa.
Gummy stem blight shows a variety of symptoms which are referred to as leafspot, stem canker, vine wilt, and black rot or fruit. D. bryoniae invades the leaves and stems of watermelon, cucumber, and muskmelon (cantaloupe). The appearance of spots on the leaves, petioles and stems are a typical sign of infection which usually become pale brown or gray. Spots on stems often elongate into streaks (usually starting at the joints) and have an amber exudate of gummy material. Leaves may turn yellow and die; occasionally the whole plant wilts and turns brown. As spots get older, they become lesions. During the rainy season lesions can become watersoaked and can spread and lead to severe defoliation. Further development can lead to bark scaling and cracking in cucurbit vines and the collar region of watermelon. Gummy exudates may occur from cracks, especially in watermelon and pumpkin. Severe infection often results in death of the plant.
D. bryoniae invades the fruit of squash, pumpkin, and gourd and initially appears as spots on the surface of the fruit. The size and coloring of spots vary according to crop. They are irregularly circular, first appearing as a fading of the fruit color then they eventually turn to gray or brown. These darkened spots may have a hardened droplet of a gummy substance in the center. The spot later turns black and may penetrate through the rind (it is now a lesion). The presence of the lesion provides access for other organisms to invade the fruit and can cause wet-rot, where the whole fruit may become a watery mass enclosed by the rind.
The chief diagnostic sign of the spots, whether on fruit, stem, or leaf, is the presence of the closely spaced groups of the dark brown to black fruiting bodies, just large enough to be seen without a hand lens. Sometimes these are arranged in rings on the fruit or leaf surface. While the pathogen usually attacks the leaves and stems of some hosts and the fruit of others, when conditions are ideal all parts of all hosts are susceptible.
Infected watermelon fruit flesh is pale-pink and watery in taste; pumpkin fruits display poor cooking qualities. Stored crops may also become infected.
D. bryoniae (syns.: D. melonis; Mycosphaerella citrullina; and M. melonis). M. melonis is a facultative necrotroph (lives mostly on dead tissue, but not required to). Facultatively necrotrophic fungi are often dependent on host surface exudates prior to penetration and lesion formation (Svedelius, 1990). While uninjured tissues have been shown to become infected when exogenous nutrients are present (Bergstrom et al., 1982), mechanical injuries and bruises can guarentee disease development (especially in fruit).
The fungus produces great numbers of pale-colored pycnidia and dark globular perithecia either submerged, partly exposed, or wholly on the plant surface. The pycnidia are filled with hyaline, mostly septate spores (conidia) that are forced out in long cream to pink tendrils. When water dissolves the gelatinous matrix, they (the spores) are scattered, usually by wind and rain, and after about a week cause new centers of infection. The sexual stage of the fungus is represented by the perithecia that may accompany the pycnidia or come later. These are crowded with asci, each bearing eight two-celled spores (ascospores) that also serve as inoculum. Neither spore form (conidia, ascopores) is able to survive long after it is set free. The fungus lives between crops in diseased plant refuse (as chlamydospores) and possibly in or on the seed. It is shipped long distances on infected fruits. If the decayed portion of a shipment is dumped into a field or possibly on a manure pile that later is scattered over fields, the pathogen has every chance for dissemination. It may also be possible that the fungus lives in weed hosts where it is able to reproduce (Chupp and Sherf, 1960; Agrios, 1988).
The temperature range for fungal growth appears to be similar for in vitro cultures and field examinations. The fungus is able to grow within a range of 7 C to 33 C. The optimal temperature for disease development ranges from 20 C to 28 C. Moisture is necessary for fungal growth; during periods of frequent rainfall numerous dark-brown, ostiolate, partially immersed pycnidia or black perithecia are produced in lesions (Chupp and Sherf, 1960; Punithalingam and Holliday, 1972; Luepschen 1961; Chiu, 1948).
Phyllosticta orbicularis Ellis and Everhart, P. cucurbitacearum
Saccardo, P. sechiae E. Young, and P. citrullina Chester, described as cucurbit pathogens, probably are the pycnidial form of Didymella (Chupp and Sherf, 1960).
D. bryoniae is spread by the transfer of conidia which can occur through air transport or water splashing. In addition, the fungus can survive in dead plant tissue and infect subsequent crop plantings. Reports on seed transmission are conflicting; cucumber seeds can be inoculated successfully, however, there is no evidence that this occurs naturally (Punithalingam and Holliday, 1972).
Striped cucumber beetles (Diabrotica undecimpunctata howardii Barber and Acalymma vittatum Fabricius) are believed to transmit D. bryoniae in a nonpersistent manner. The beetles injure healthy plants which provide the opportunity for infection. Plants injured by beetles and then inoculated with the fungus developed symptoms in 3 to 7 days and all lesions were exclusively around the sites of beetle injury. Uninjured plants inoculated with the fungus did not develop disease symptoms. In addition, cucumber plants infested with melon aphids were susceptible to the fungus as were plants infected with powdery mildew (Erysiphe cichoracearum DC. or Sphaerotheca fuliginea (Schlect.) Poll.) (Bergstrom et. al., 1982).
Long rotations with crops other than cucurbits (at least 18 months between cucurbit crops) and proper soil drainage are important in the control of this disease. Wounding fruit during harvest and in storage must be avoided. Curing squash at 23-29 C for two weeks to heal any wounds (but not bruises) and subsequent storage at 10-12 C until fruit reaches the market are good precautionary measures.
There are Didymella resistant varieties of a few cucurbit crops, however, these varieties are often not resistant to all major diseases. Thus, careful control and harvesting practices remain necessary. Four cantaloupe varieties are reported to be resistant to gummy stem blight: Chilton, Gulfcoast, AUrora, and AC-70-154, the latter of which has only been tested in greenhouse conditions. Each cultivar, when exposed to the fungus, had less than 20% injury (Norton and Cosper, 1989). No high resistance varieties are reported for watermelon, though some U.S. varieties show some resistance which is similar to that seen in cantaloupe (Punithalingam and Holliday, 1972).
Control is difficult, often requiring the use of frequent applications of such fungicides as mancozeb, chlorothalonil, and captan. Good control of the leaf and stem infections reduces fruit infections both in the field and in storage.
Controlling for anthracnose (Colletotrichum lagenarium) in watermelon also controlled Didymella bryoniae. Benlate and related compounds were effective and Dithane M-45 was also consistently good. Although no signs of tolerance by the fungus to Benlate have been detected so far, Dithane M-45 is now recommended to be used alternately (Peregrine et al., 1984).
Chemical control with dithiocarbamates, mercurials and Cu may be effective. Chemical control is not effective during periods of high humidity and rainfall (Norton and Cosper, 1989).
A study from Greece reported resistance of D. bryoniae to benzimidazole fungicide in plastic greenhouses in Crete in the first year of application. While resistance to benzimidazoles by fungi has been reported, it usually appears 2-3 years after intensive use. The investigation in Greece showed that D. bryoniae began to resist the fungicide after only one year and the application was not intensive. One possibility is that the fungus was not detected early enough and when benzimadazoles were applied, the disease had already progressed too far for control. Benzimadozoles can be replaced by carbamates, triforine and iprodione. (Malathrakis and Vakalounakis, 1983).
Post-harvest control can be achieved by dipping squash, as soon as they are picked and dipped for an instant in either formaldehyde (diluted with water 1:50) or in Chlorox, 1 quart in 50 gallons of water. They are then placed in a curing room. The dipping does reduce rot when the crop is poorly sprayed and when the harvesting is done carelessly. When other parts of the control program are done well, dipping is not considered useful (Chupp and Sherf, 1960).
Agrios, G.N. 1988. Plant Pathology, 3rd ed. Academic Press, Inc. New York. 803 pp.
Bala, G. and F. Hosein. 1986. Studies on gummy stem blight disease of cucurbits in Trinidad. Trop. Agric. 63:195-197.
Bergstrom, G.C., D.E. Knavel, and J. Kuc. 1982. Role of insect injury and powdery mildew in the epidemiology of the gummy stem blight disease of cucurbits. Plant Disease 66:683-686.
Chiu, W.F. 1948. The pathogenicity of Mycosphaerella citrullina. Phytopathology 38:5.
Chupp, C. and A.F. Sherf. 1960. Pp 314-317. Vegetable diseases and their control. The Ronald Press Company, New York. 693 pp.
Luepschen, N.S. 1961. The development of Mycosphaerella black rot and Pellicularia rolfsii rot of watermelons at various temperatures. Plant Disease 45:557-559.
Malathrakis, N.E. and D.J. Vakalounakis. 1983. Resistance to benzimidazole fungicides in the gummy stem blight pathogen Didymella bryoniae on cucurbits. Plant Pathology 32:395-399.
Norton, J.D. and R.D. Cosper. 1989. AC-70-154, a gummy stem blight-resistant muskmelon breeding line. HortScience 24:709-711.
Peregrine, W.T.H., K. Ahmad, and M. Momin. 1984. Controlling anthracnose in watermelon. World Crops 36:184-185.
Punithalingam, E. and P. Holliday. 1972. Didymella bryoniae. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 332.
Svedelius, G. 1990. Effects of environmental factors and leaf age on growth and infectivity of Didymella bryoniae. Mycological Research. 94(7): 885-889.
van Steekelenburg, N.A.M. 1986. Factors influencing internal fruit rot of cucumber caused by Didymella bryoniae. Netherlands J. Plant Pathology 92:81-91.