Pathogen Biology

Monosporascus cannonballus is a pyrenomycete ascomycete fungus. It forms large (40-50 µm-diam), spherical, multi-layer walled ascospores (Figures 6 & 7), however typically only one ascospore is formed per ascus instead of the usual eight (Figure 8). Ascospores are multinucleate, typically having eight nuclei, although occasionally they may have 16. There is no known anamorph (asexual) stage. Two species have been described, M. cannonballus and M. eutypoides. Both are pathogenic to melon, and they were originally distinguished from each other on the basis of the number of ascospores per ascus. However, later studies based on partial DNA sequence similarity, as well as other phenotypic traits, suggest that they are conspecific (the same species). Since M. cannonballus is the original species name (holotype), it has been retained.

Figure 6.	Figure 7.	Figure 8.

The fungus is homothallic (self-fertile) and readily forms fertile perithecia in host root tissue (Figures 3 and 4) and in vitro on artificial growth media (Figure 5). Ascospores are thick, multi-walled spores (Figures 6, 7, and 8) and are extremely resistant to desiccation and other factors. Germination of the ascospore is rare in vitro, however germination is enhanced in situ when in the presence of root exudates from growing muskmelon seedlings.

Figure 3.	Figure 4.	Figure 5.

Monosporascus cannonballus is adapted to hot, dry climates. In vitro vegetative growth is optimal at 25 to 35°C (77 to 95°F), while perithecia are formed most readily at 25 to 30°C (77 to 86°F). Mycelial growth occurs over a pH range of 5 to 9, but is optimal from pH 6 to 7 and inhibited completely at pH 4 and below. Monosporascus cannonballus appears also to be adapted to slightly or moderately alkaline and saline soils. The fungus grows readily on several standard laboratory growth media (e.g. potato dextrose agar, V-8 juice agar, and water agar) and forms fertile black perithecia within 2 to 3 weeks. Perithecia are readily visible against the light gray or dirty white mycelium.

There appears to be little genetic variation within M. cannonballus. Although a detailed analysis of a large population has not been done, selected isolates of M. cannonballus from several countries, including the U. S., Japan, and Spain, were identical in colony morphology, number of ascospores/ascus (one), virulence to melon, and DNA sequence across the internal transcribed spacer regions (ITS 1 and 2) and 5.8S gene of the rDNA. This latter fact is a strong indication that the isolates are genetically very similar or identical. As a result, a PCR-based protocol using species-specific ITS primers has been developed for the early detection of M. cannonballus in root tissue. There do appear to be some differences in vegetative compatibility among U.S. isolates and those from other regions of the world. This aspect has not been studied in depth and could add valuable information to understanding the genetics of this fungus. In some instances, extreme variations in colony appearance, including the formation of dark yellow, orange or brown pigments (Figure 9), reduced vegetative growth, and loss of perithecia production, as well as loss of virulence and colony death, have been reported. Isolates displaying one or more of these phenotypes were shown to harbor one or more double-stranded RNAs (dsRNA), which were attributed to causing the aberrant phenotype. This has been explored as a potential biocontrol of Monosporascus root rot and vine decline, which is described further in the Management section.

Figure 9.

BACK TO TOP

Copyright © 2002
by The American Phytopathological Society