''Didymella bryoniae,'' syn. ''Mycosphaerella melonis,'' is an ascomycete fungal

plant pathogen Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...

that causes gummy stem blight on the family ''Cucurbitaceae'' (the family of gourds and melons), which includes cantaloupe, cucumber, muskmelon and watermelon plants. The anamorph/asexual stage for this fungus is called ''Phoma cucurbitacearum''. When this pathogen infects the fruit of cucurbits it is called black rot.

Host symptoms

* Gray-green to black circular leaf spots * Angular/target-like water-soaked lesions * Stem lesions/cankers * Vine lesions * Vine Necrosis * Reddish gummy ooze exuding from the lesions/wounds * Wilt * Defoliation The first symptoms appear as grayish green, circular spots between the veins of the leaf lobes. With age these spots darken to brown and black. 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. The petioles and stems eventually become necrotic and often shrivel. Eventually all infected vines will become necrotic and occasionally the plant dies due to wilting and defoliation. Another common sign following the stem lesions is a red to amber colored ooze. Some regions report the presence of small pseudothecia as black specks inside the cankers. Gummy stem blight can be confused with

anthracnose A plant canker is a small area of dead tissue, which grows slowly, often over years. Some cankers are of only minor consequence, but others are ultimately lethal and therefore can have major economic implications for agriculture and horticultur ...

, which is caused by a fungal plant pathogen called '' Colletotrichum lagenarium''. To distinguish between anthracnose and gummy stem blight, gummy stem blight leaf lesions are darker, target-like and less deteriorated than anthracnose lesions. Newly infected plants will begin to show symptoms within 7–12 days.

Signs

* Black specks ( Perithecia and Pycnidia) on cankers * Round-ended, cylindrical, monoseptate and

hyaline A hyaline substance is one with a glassy appearance. The word is derived from el, ὑάλινος, translit=hyálinos, lit=transparent, and el, ὕαλος, translit=hýalos, lit=crystal, glass, label=none. Histopathology Hyaline cartilage is ...

conidia **

Conidia A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...

dimensions: 6.4–13.6 μm in length and 3.69–4.68 μm in diameter In vitro, the fungal growth on an agar plate looks rough and undulated. When grown in vitro on agar, the fungus produces a white to olive-colored mycelium. In latter periods of growth, the mycelium is an olive to dark green or black color.

Disease cycle

''Didymella bryoniae'' survives on deceased vines, crop debris and on seeds in between seasons and ''D. bryoniae'' can survive for 5 months on the soil surface in winter. The fungus develops best under moist conditions, and cotyledons and young watermelon/melon leaves are especially susceptible to the fungus. ''D. bryoniae'' produces ascospores (meiotic spores) in perithecia and conidia (mitotic spores) in pycnidia and both of these spores are dispersed by rain/rain-splash and UV light is needed in order for the fungus to sporulate. Ideal ascospore dispersal occurs after nightly rainfall and dew periods. In order to infect, ascospores must land on leaves that have free-standing water on them. Next the ascospores penetrate through the leaf cuticle. Stems may be infected by ''D. bryoniae'' ascospores through stem wounds or by the extension of leaf lesions. Fruits are penetrated through wounds and pollination flower scars. Conidia are produced on the lesion sites of leaves and stems. Certain '' Cucurbita'' species are resistant to ''D. bryoniae'' but become vulnerable once they mature.

Epidemiology

''Didymella bryoniae'' is common in the Southern U.S. and other subtropical or tropical locations. Most infections occur during rainy/wet seasons, in which the humid is greater than 90% and the temperature is roughly 20–24 °C. Humidity seems to be a larger factor than temperature when it comes to infection success. ''D. bryoniae'' can also be found in temperate regions, especially where winter squash and pumpkins are grown. This pathogen is also common in greenhouses where cucumbers are grown. ''D. bryoniae'' can be spread by the transfer of conidia through a variety of fashions. The most common forms of transfer for these conidia are through the air and water splashing. The fungus is capable of surviving in dead plant tissue giving it the ability to infect the following crop planting. The pathogen requires an entry site on the plant in order to infect so areas that also experience issues with pests are at higher risk. In vitro, ''D. bryoniae'' does not form pycnidia without UV-light but if cultured in the presence of UV light and darkness, conidia/

pycnidiospore {{Short pages monitor