Abstract

The ascomycete Monosporascus cannonballus Pollack et Uecker is one of the mean fungal agents involved in “root rot and vine decline” or “collapse”, that affects cucurbitaceous crops all over the world and limits their production in Spain. Since the early eighties, problems associated with this fungus have increased on muskmelon and watermelon in the mean cucurbit growing areas in Comunidad Valenciana.

Ascospores are the main inoculum of M. cannonballus, and remain in the soil after decomposition of the affected roots. These ascospores can be recovered using a physical extraction method, that allows their quantification and monitoring over the time. In this Thesis, different epidemiological studies have been done using this technique, making possible to obtain innovative results.

In the first study, the population dynamics of ascospores of M. cannonballus in soil under different crop conditions was studied. In muskmelon fields, the maximum level of ascospores in soil was observed seven months after plantation (3-4 months after the end of the cropping season and death of plants). Later, twelve moths after plantation, this level decreased progressively, reaching a level similar to the initial one. In fields with seasonal flooding (in winter) and without melon cropping, a slow and progressive decrease of the ascospore level was observed. In this case, we verified that ascospores of M. cannonballus can survive in the soil at least for a 3-year period, maintaining their capability to infect muskmelon roots. M. cannonballus is considered a termophilic fungus, adapted to hot and semiarid climates. In this study, we demonstrated that M. cannonballus can survive in temperate climates and under flooding conditions.

The population of ascospores in the soil of muskmelon fields in Comunidad Valenciana has been quantified. Ascospores of M. cannonballus were detected in all of studied fields. In the samples taken after the apparition of symptoms of “collapse”, significant differences between fields and between symptomatic and asymptomatic zones were observed. There were no significant differences in the row position (in-bed and between-bed zones). Likewise, the comparison between the initial level of ascospores in soil with the level obtained two or three months after the end of the cropping season, showed different situations, sometimes contradictories: in some fields there was an increase in the final level of ascospores per gram of soil, but in others, a decrease of this level or no variation was observed. As a consequence, this makes evident the need of using a sequential sampling to better study the population dynamics of ascospores of M. cannonballus in soil. Because of the complexity of the life cycle of M. cannonballus (germination of ascospores, colonization of the roots, formation of perithecia and incorporation of the new ascospores into the soil), punctual sampling does not provide enough information concerning the inoculum level of this soil-borne pathogen in a growing season.

At the same time, the population of ascospores of M. cannonballus in soil, was studied in several cucurbitaceous crops (muskmelon, watermelon and grafted watermelon onto Cucurbita rootsrocks). The population dynamics of ascospores was different in every studied crop. In muskmelon and watermelon crops a decrease in the level of ascospores was observed since the first sampling, due probably to the germination of ascospores and the infection and colonization of the roots. This decrease coincided with the observation of the first symptoms on the canopy and with the increase of the isolation of M. cannonballus in the affected roots. Subsequently, it was observed an increase in the population of ascospores in soil two months after the plantation, due probably to the production of perithecia in the infected roots. This increase in the level of ascospores in the final moments of the crop, was significantly higher in-bed that between-bed position, associated probably to a higher density of roots. In grafted watermelon, the level of ascospores of M. cannonballus in soil remained stable during the crop. Significant differences in the row position, neither symptoms in the canopy, nor formation of perithecia in the roots were observed. Moreover, the percentage of isolation was very low during all the sampling process. According to these results, we consider that grafting watermelon onto Cucurbita rootstocks is an efficient method to control the “collapse” caused by M. cannonballus.

Finally, we set up a pathogenicity test of M. cannonballus, Acremonium cucurbitacearum Alfaro-García, W. Gams et J. García-Jiménez, Rhizopycnis vagum D.F. Farr and Plectosporium tabacinum (van Beyma) M.E. Palm, W. Gams et Nirenberg. In this test we considered all the possible combinations among them. There was no positive correlation between root damage index (RDI) and the dry weight of the canopy. Thus, this parameter is not appropriated to evaluate the pathogenicity of these fungi in greenhouse tests, because the plants can not fructify and the collapse does not occur. The inoculations which caused higher DRIs were those with M. cannonballus and their combinations, while the inoculations with lower DRIs  were those with R. vagum, P. tabacinum and their duplex combination. It has been demonstrated the importance of M. cannonballus and A. cucurbitacearum as causal agents of muskmelon “collapse”.