Cucurbit Root Rot: Learn About Monosporascus Root Rot Of Cucurbits

Cucurbit monosporascus root rot is a serious disease of melons, and to a lesser extent other cucurbit crops. A fairly recent problem in melon crops, cucurbit root rot loss can run from 10-25% to 100% in commercial field production. The pathogen can live in soil for a number of years, making cucurbit monsporascus treatment difficult. The following article discusses monosporascus root rot of cucurbits and how to manage the disease.

What is Cucurbit Monosporascus Root Rot?

Cucurbit root rot is a soil borne, root infecting fungal disease caused by the pathogen Monosporascus cannonballus that was first noted in Arizona in 1970. Since then, it has been found in Texas, Arizona, and California in the United States, and other countries such as Mexico, Guatemala, Honduras, Spain, Israel, Iran, Libya, Tunisia, Pakistan, India, Saudi Arabia, Italy, Brazil, Japan, and Taiwan. In all of these regions, the common factor is hot, arid conditions. Also, the soil in these areas tends to be alkaline and containing significant salt.

Cucurbits affected by this pathogen are small in size with low sugar content and are susceptible to sun scald damage.

Symptoms of Monosporascus Root Rot of Cucurbits

Symptoms of M. cannonballus are not usually visible until near harvest time. Plants yellow, wilt and leaves dieback. As the disease progresses, the entire plant dies prematurely.

Although other pathogens result in similar symptoms, M. cannonballus is notable for its reduction in length of infected vines and the absence of lesions on visible plant parts. Also, roots infected with cucurbit root rot will have black perithecia visible in the root structures that appear as small black swellings.

Although uncommon, on occasion, vascular browning is present. Areas of the taproot and some lateral roots will show darkened areas that may become necrotic.

Cucurbit Monosporascus Treatment

M. cannonballus is transmitted via the planting of infected seedlings and the replanting of cucurbit crops in infected fields. It is unlikely that it is transmitted by water movement such as heavy rain or irrigation.

The disease is often indigenous to the soil and is fostered by continued cucurbit cultivation. Although soil fumigation is effective, it is also costly. Cucurbits should not be planted in areas with proven consistent infection of this disease. Crop rotation and good cultural practices are the best non-control methods for the disease.

Fungicide treatments applied just at plant emergence have been shown to be affective at controlling Monosporascus root rot of cucurbits.

Management of Monosporascus sudden wilt of melon by soil application of fungicides

Sudden wilt (vine decline) of melon caused by Monosporascus cannonballus is a problem in arid and semiarid regions worldwide. Preplanting soil disinfestation with methyl bromide, a common treatment for disease management, has been banned in many countries, raising the need for alternative disease-control measures. Soil fungicide application during the growing season is one possible treatment. Twelve fungicides were evaluated in vitro for M. cannonballus suppression, seven of those were evaluated under field conditions. The fungicides azoxistrobin, prochloraz and pyraclostrobin + boscalid exhibited high and similar efficacies in controlling sudden wilt disease under field conditions. Fludioxonil applied at high rates was also effective but was phytotoxic. Fluazinam, the first fungicide found capable of suppressing sudden wilt and one which has been used in Israel since 2000, was less effective. The results indicate that two applications of a fungicide during the short fall season should be sufficient for effective control of the disease. In the long spring season, at least three applications are needed to protect the melon crop. Melon fruits were examined for fungicide residues and only boscalid residues were found. This fungicide was therefore limited to the first application before fruit set.

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Symptoms and signs

White, powdery fungal growth develops on both leaf surfaces, petioles, and stems. This growth is primarily asexual spores called conidia. It usually develops first on crown leaves, on shaded lower leaves and on leaf undersurfaces. Yellow spots may form on upper leaf surfaces opposite powdery mildew colonies. Older plants are affected first. Infected leaves usually wither and die. Plants may senesce prematurely. Fruit infection occurs rarely on watermelon and cucumber. Cleistothecia are dark brown, small (diameter of about 0.003 inches) structures that are barely discernable without a hand lens. They develop late in the growing season. The sexual spores within these structures are protected from adverse conditions.

See also: LIHREC Cucurbit powdery mildew photo gallery (includes diagnostic images)

Cucurbit Monosporascus Treatment - Tips For Managing Cucurbit Monosporascus Root Rot - garden

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Interpretive Summary: Muskmelon is an important horticultural crops in the U S comprising more than 40,000 hectares at a value of approximately $125 million. Almost 90% of the production is in Arizona, California, and Texas. The soilborne pathogen Monosporascus cannonballus is the greatest yield-limiting factor to muskmelon production in these states causing severe losses in some years. Currently, there are no effective control measures for Monosporascus root rot/vine decline available and genetic resistance is inadequate in commercial cultivars. Biological control of Monosporascus root rot/vine decline of muskmelon and watermelon provides a promising strategy to explore since biocontrol agents can colonize plant root systems and interact with the pathogens to suppress root diseases. Based on this concept, we attempted to determine the potential of a biocontrol agent, Trichoderma virens, for the control of Monosporascus root rot/vine decline of muskmelon plants. Our experimental results indicated that T. virens ha both antibiotic and parasitic activity against M. cannonballus. Under greenhouse conditions, T. virens colonized muskmelon seedling root system, and reduced colonization of muskmelon roots by M. cannonballus, and significantly suppressed disease severity after a seed treatment with T. virens preparations. This is the first time that T. virens is demonstrated to have a potential for the control of root rot/vine decline of muskmelon. Our experimental results appear to provide a new strategy to implement into the integrated management of root rot/vine decline diseases of cucurbits.

Technical Abstract: Root rot/vine decline disease of muskmelon, incited by Monosporascus cannonballus, causes severe economic losses in many hot semi-arid production areas. No effective control measures for Monosporascus root rot/vine decline are presently available and genetic resistance is not adequate in commercial cultivars. We have attempted to determine the potential for biocontrol of root rot/vine decline diseases on muskmelon using Trichoderma virens. T. virens exhibited in vitro antibiotic activity by inhibiting mycelial growth of M. cannonballus and other vine decline pathgens such as Didymella bryoniae, Macrophomina phaseolina and Phomopsis cucurbitae. A gliotoxin producing strain of T. virens (TV-6) demonstrated much stronger inhibition of the fungal pathogens when compared with T. virens strain TV-4 which does not produce gliotoxin. In addition to antibiotic activity, T. virens demonstrated parasitism of M. cannonballus mycelium. Under greenhouse conditions, T. virens colonized the root systems of muskmelon plants, significantly reduced the colonization by M. cannonballus of muskmelon seedling roots, and suppressed severity of seedling disease using a seed treatment. T. virens strains TV-4 and TV-6 performed best in suppression of M. cannonballus root rot among the T. virens strains and isolates tested. Preliminary experimental data suggest that strains of the biocontrol agent T. virens may have potential as an additional strategy for the integrated management of root rot/vine decline diseases of muskmelon.

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