Monographella cucumerina (Lindf.) Arx, (1984) 

Plectosporiose (Plectosporium blight, Microdochium blight)

General  

• A fungus widely distributed worldwide and reported on various Cucurbitaceae (hubbard squash, buttercup squash, mammoth squash, pumpkin, yellow squash, zucchini and butternut squash - Cucurbita moschata - and pumpkin - Cucurbita maxima )* in a rather limited number of production areas located in the United States, Asia and Europe in particular.
• Observed in the open field, with a distribution of foci in the plots.

• Susceptible botanical family(ies)*

• Production areas affected :

• Organs attacked

* In France, we have quite systematically observed and isolated it on the roots of tomatoes grown in the ground, but especially above ground. It is in this latter cultural context that it finds particularly favorable conditions for its development. It is isolated on the roots with very high frequencies at certain times of the year, higher than those obtained for Fusarium oxysporum. Its parasitic status in soilless crops is still poorly defined. It was also described in Egypt in 1981 as responsible for root rot, yellowing and leaf wilting followed by tomato seedling mortality. We have also associated it with lesions at the collar on young tomato plants in France. He also reported it on tomato, basil, lupine, sunflower, groundnut, sugar beet, bellflower. It should be noted that it has been isolated in many soils around the world and on the roots of the most varied hosts. 

Symptoms and signs

• Symptoms :
  • Small white to creamy elongated to diamond-shaped lesions developing on leaf blades (Figures 1 and 2) and leaf veins (Figures 3 to 5), petioles (making them brittle) (Figure 6), twigs (Figure 7 ), and fruit peduncles. Severe attacks can lead to the death of leaves and even plants.
  • Comparable lesions on fruits, rather circular to irregular, white or even metallic in color (tanned to silvery). As they evolve, they merge and make the surface of the fruits more or less dry and speckled. It should be noted that the lesions on the fruits allow the invasion of secondary bioaggressors responsible for wet rots.

• >>> More pictures  

• Signs : although sporulating on the altered tissues, no signs are visible to the eye. Microscopic observations make it possible to visualize the presence of slender phialides, slightly swollen at the base, on which are produced tiny spores hyaline uni- or bi-cellular conidia (figure 8). The teleomorphic or sexual form is materialized by perithecia containing asci in which hyaline bicellular ascospores are formed (figure 9).
• Possible confusion :

Biology

• Its biology is not well known.
• Conservation : frequent in dark soils, it remains there for several years, especially on plant debris. Reported in many soils around the world and on the roots of the most varied hosts in addition to Cucurbitaceae, (basil, lupine, sunflower, peanut, sugar beet, bellflower, etc.).
• Sporulation : produces numerous conidia on damaged tissues of several organs; but also more rarely perithecia materializing its sexual reproduction.
• Dissemination : its conidia are dispersed by the wind over long distances and by splashing water. This fungus is transmitted by seeds.
• Favorable conditions : would appreciate rainy and humid climates, and cool temperatures. Note that the thermal optimum for the germination of its conidia is around 25°C, and that severe attacks have been observed at temperatures between 25 and 32°C.

Protection

• Use healthy seeds .
• Rotate Cucurbits for at least three years.
• Ensure good drainage of cultivated plots.
• Avoid too high planting densities in order to favor the aeration of the foliage.
• Orient the rows of plantations in the direction of the prevailing wind.
• Strip the lower parts of the plants in order to eliminate the first affected leaves and improve the ventilation of the plant cover.
• Avoid irrigation , prefer drip irrigation. If they are essential, carry them out in the morning so that the vegetation drains quickly during the day.
• Do not allow workers to work while vegetation is wet.
• Eliminate plant residues , during cultivation following the various cultivation operations, and at the end of cultivation after uprooting the plants. They will have to be destroyed.
• No fungicide is approved for this use in France. Several fungicides are effective against this fungus.