Geslachtelijke fase[ bewerken brontekst bewerken ] Bij Didymella bryoniae wordt een ascocarp gevormd bestaande uit in elkaar gevlochten hyfen. Het pseudothecium is rond, donker tot zwart gekleurd en ligt verzonken in het plantenweefsel van stengels, bladeren en vruchten. Aan het oppervlak zit een tot 30 micrometer grote, conische verhoging, waarop de 30—55 micrometer grote opening ostiole zit. Schema van het ascocarp van Didymella bryoniae De wand van het pseudothecium is aan de zijkanten en de basis verdikt. De wand is aan de bovenzijde 18 micrometer dik, aan de zijkant 20—30 micrometer en aan de basis 25—40 micrometer.
|Published (Last):||23 May 2016|
|PDF File Size:||15.34 Mb|
|ePub File Size:||18.62 Mb|
|Price:||Free* [*Free Regsitration Required]|
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. 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. Facultatively necrotrophic fungi are often dependent on host surface exudates prior to penetration and lesion formation Svedelius, While uninjured tissues have been shown to become infected when exogenous nutrients are present Bergstrom et al.
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, ; Agrios, 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, ; Punithalingam and Holliday, ; Luepschen ; Chiu, Phyllosticta orbicularis Ellis and Everhart, P.
Young, and P. 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, Striped cucumber beetles Diabrotica undecimpunctata howardii Barber and Acalymma vittatum Fabricius are believed to transmit D. 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. Bergstrom et. Wounding fruit during harvest and in storage must be avoided. Curing squash at C for two weeks to heal any wounds but not bruises and subsequent storage at 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, the latter of which has only been tested in greenhouse conditions.
No high resistance varieties are reported for watermelon, though some U. 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 was also consistently good.
Although no signs of tolerance by the fungus to Benlate have been detected so far, Dithane M is now recommended to be used alternately Peregrine et al. 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, A study from Greece reported resistance of D. While resistance to benzimidazoles by fungi has been reported, it usually appears years after intensive use.
The investigation in Greece showed that D. 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, 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 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, Plant Pathology, 3rd ed.
Academic Press, Inc. New York. Bala, G. Studies on gummy stem blight disease of cucurbits in Trinidad. Bergstrom, G. Knavel, and J. Role of insect injury and powdery mildew in the epidemiology of the gummy stem blight disease of cucurbits. Plant Disease Chiu, W. The pathogenicity of Mycosphaerella citrullina.
Phytopathology Chupp, C. Pp Vegetable diseases and their control. Luepschen, N. The development of Mycosphaerella black rot and Pellicularia rolfsii rot of watermelons at various temperatures. Malathrakis, N. Resistance to benzimidazole fungicides in the gummy stem blight pathogen Didymella bryoniae on cucurbits. Plant Pathology Norton, J. AC, a gummy stem blight-resistant muskmelon breeding line. HortScience Peregrine, W. Ahmad, and M. Controlling anthracnose in watermelon.
World Crops Punithalingam, E. Didymella bryoniae. Svedelius, G. Effects of environmental factors and leaf age on growth and infectivity of Didymella bryoniae. Mycological Research. Factors influencing internal fruit rot of cucumber caused by Didymella bryoniae.
Crestamento gomoso do caule
With age these spots darken to brown and black [1,2]. Lesions begin to develop on vines at the vine nodes and then elongate into water-soaked streaks, and these streaks are pale brown at first but turn gray with time . Eventually all infected vines will become necrotic and occasionally the plant dies due to wilting and defoliation [1,7]. Gummy stem blight can be confused with anthracnose , which is caused by a fungal plant pathogen called Colletotrichum lagenarium .
Pseudothecien[ Bearbeiten Quelltext bearbeiten ] Das Pseudothecium ist kugelig, dunkel bis schwarz, in das Gewebe der Wirtspflanze eingesunken und dann hervorbrechend. Die Wand der Pseudothecien ist an den Seiten und an der Basis verdickt. Die Wand ist an der Oberseite 18 bis 21 Mikrometer stark. Sie sind ungestielt oder kurz gestielt und gerade oder gebogen. Pseudoparaphysen[ Bearbeiten Quelltext bearbeiten ] Neben den Asci enthalten die Pseudothecien noch sogenannte Pseudoparaphysen , die gemeinsam mit den Asci das Hymenium die Fruchtschicht des Pilzes bilden.