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Hosts

Muskmelon (Cucumis melo L. var. reticulatus) and cantaloupe (C. melo L. var. cantalupensis Naud.) are the most common reported hosts of M. cannonballus. Cucumber (C. sativus) and red clover (Trifolium pratense) also are known hosts to this pathogen. Watermelon (Citrullis vulgaris Schrad.) is a recently discovered host (pers. observ.).

A fungal specimen from an alfalfa stem (Medicago sativa) may be M. cannonballus, however the hosts' original location is unknown and the collection consists of one stem only.

Distribution

The first published report of M. cannonballus came from Arizona in 1970. It was reported as an unknown fungus from rotted secondary roots of cantaloupe plants; characterization came in 1974. It has since been reported from Texas, Japan, and India. An unpublished report suggests that M. cannonballus is present in California (Mertely et. al., 1991) and ascospores have been isolated from soil collected from watermelon fields in Hawaii, though perithecia were not seen on watermelon roots (pers. observ.).

All reported Monosporascus collections have come from relatively hot, dry areas of the world. Soils in these areas tend to be alkaline and have accumulated salts.

Symptoms

The first noticeable above-ground symptoms are gradual yellowing and dieback of the oldest leaves as plants approach maturity. Foliar deterioration advances distally as the season progresses and affected vines and plants decline and die prematurely. While vine decline is also characteristic of other pathogens, Monosporascus is distinguishable also by a significant reduction of primary vine lengths by as much as 35% in comparison to other vine reducing pathogens, the virtual absence of vascular discoloration or lesions in above-ground plant parts, and the presence of black perithecia imbedded in the roots. Perithecia are visible to the naked eye, appear as small black bulges in the root cortex, and can be considered a diagnostic trait in that they are unlikely to be confused with anything else. Infected young seedlings and recently infected plants often do not show any symptoms, including perithecia, but the fungus is recoverable from roots (Mertely et al., 1991).

Other root symptoms include a darkening of areas of the taproot and some lateral roots. These may become necrotic and slough off as the plant is pulled from the ground. The cortical tissues of affected roots often exhibit distinct lesions, sometimes sunken and usually brown to red. Brownish discoloration of the vascular tissues also occurs but rarely extends more than a few centimeters above the soil line (Mertely et al., 1991).

Fruit from fields infested with Monosporascus are often unmarketable because of their small size, low sugar content, or sun scald damage (Mertely et al., 1991).

Biology

M. cannonballus is in the class Ascomycetes, indicating that it produces sexual spores called ascospores. Ascospores are produced within an asci, which has a layer of differentiated hyphae around it, the perithecial wall. This entire unit is the perithecia (Uecker and Pollack, 1975). Perithecia of M. cannonballus are imbedded in or emergent on host roots. Small necrotic roots 1-3 mm in diameter often support large numbers of perithecia which are visible to the naked eye as small black bulges in the root cortex.

Ascomycetes, in general, produce eight ascospores within an ascus, however, M. cannonballus produces one ascospore in each ascus (and rarely two). Ascospores are released approximately 7 hours after the asci are released from the perithecium (Uecker and Pollack, 1975). In a laboratory environment, M. cannonballus grows rapidly at 24°-38° C on a range of media and form perithecia abundantly at 28°-29° C on malt-yeast extract agar after 1 month (Sivanesan, 1991). Ascospore germination has been observed, in vitro, after heat-treating 6-month-old pores to 45° C for 10 minutes; maximum germination was less than 0.05%. In addition, a 5% Chlorox treatment for 4 minutes also resulted in some germination, but at a lower rate than by heating (Martyn et al., 1992). There is no information regarding the necessary environmental conditions (if any) for the release of asci or ascospores under field conditions. Asexual spores (conidia) have not been observed for this pathogen.

Isolates of M. cannonballus vary in aggressiveness and virulence. This poses a problem in assessing field infestation levels in that it is not known at what levels a fungal population in a given field is pathogenic or nonpathogenic. One study tested a total of eight isolates of M. cannonballus. Four isolates were aggressive pathogens in greenhouse tests and also showed high disease incidence in the laboratory seedling tests (TX(Cm)90-23, -24, -25, -30). The other four displayed reduced pathogenicity (TX(Cm)90-26, -27, -28, -29). Another study examined healthy muskmelon seedlings transplanted to artificially infested soil with one of two M. cannonballus isolates (CH 83-14 and CH 83-15) of M. cannonballus. Plants collapsed after 50 days and the fungus was recovered from roots.

Epidemiology

While there is no information in the literature regarding the spread of the pathogen, M. cannonballus apparently is able to survive in soil for some time. Uematsu and Sekiyama (1990) reported that melon plants grown in infested field soil samples, which had been in storage for 1-5 years, developed root rot symptoms and the pathogen was isolated from the roots. The soil samples consisted of 13 M. cannonballus isolates collected from various areas of Japan.

It is likely that this pathogen is spread similarly to other root pathogens, by means of transplanting infected seedlings, replanting susceptible crops in infected fields, etc.

Management

NON-CHEMICAL CONTROL

There is no information in the literature regarding non-chemical control of M. cannonballus. As stated above, it appears that once the pathogen is in the soil that it is pathogenic for a number of years (Uematsu and Sekiyama, 1990). A separate study reported an association between successive cropping practices and increased inoculum density (Mertely et al., 1992). Crop rotation and good cultural practices are likely to be the best non-control methods.

There is no information regarding resistant cultivars while susceptible muskmelon cultivars that have been mentioned in the literature are Laguna, Durango and Magnum 45 (Mertely et al., 1991).

CHEMICAL CONTROL

Busan 1020, Telone C17 and Telone II have been tested for their efficacy in increasing muskmelon yield from M. cannonballus infested fields. In 1989 and 1990 the fumigants were injected into the center of muskmelon beds in 0.3 m treatment zones. At all application rates and soil depths, each fumigant-treated bed had a significantly increased yield over nonfumigated control beds. Telone C17 and Telone II showed an increase in yield with increasing application rates, Busan 1020 did not show a rate response. In 1991, the fumigants were applied to beds in 0.3, 0.6, and 0.9 m treatment zones to determine if extending the zones would increase the efficacy of the fumigants. No significant yield increase was observed (Miller et. al., 1992).

References

Martyn, R., J. Mertely, M. Miller, C. Katsar, and R. Baasiri. 1992. Morphology and germination of Monosporascus cannonballus ascospores. APS/MSA Joint Meeting, Abstracts of Presentations.

Mertely, J.C., R.D. Martyn, and M.E. Miller. 1992. Quantification of Monosporascus cannonballus ascospores in commercial muskmelon fields in South Texas. APS/MSA Joint Meeting, Abstracts of Presentations.

Mertely, J.C., R.D. Martyn, M.E. Miller, and B.D. Bruton. 1991. Role of Monosporascus cannonballus and other fungi in a root rot/vine decline disease of muskmelon. Plant Disease 75:1133-1137.

Miller, M.E., R.D. Martyn, and B.D. Bruton. 1992. Yield of muskmelon (Cucumis melo) as affected by fumigants in fields infested with Monosporascus cannonballus. APS/MSA Joint Meeting, Abstracts of Presentations.

Sivanesan, A. 1991. Monosporascus cannonballus. IMI Descriptions of Fungi and Bacteria, No. 1035. Mycopathologia 114:53-54.

Uecker, F.A., and F.G. Pollack. 1975. Development and cytology of Monosporascus cannonballus. Bot. Gaz. 136:333-340.

Uematsu, S., S. Onogi, and T. Watanabe. 1985. Pathogenicity of Monosporascus cannonballus Pollack and Uecker in relation to melon root rot in Japan. Ann. Phytopath. Soc. Japan 51:272-276.

Uematsu, S. and K. Sekiyama. 1990. Comparison of morphological characteristics and pathogenicity of Monosporascus cannonballus Pollack and Uecker collected in Japan, distribution in melon plants with root rot symptoms and survival in soils under laboratory conditions. Soil Microorganisms 35:7-12.

JUNE 1992

2A-MOCAN

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