Isolation and molecular identification of melon (Cucumis melo) rhizosphere inhabitant Actinomycetes with plant growth promoting activity under biotic stress caused by Macrophomina phaseolina

Melon charcoal rot is one of the most important diseases that significantly reduces the yield of melon (Cucumis melo L.). The main objectives of this study were: isolation of actinomycetes from the rhizospheric soil of different melon cultivation regions located in the Kerman and Sistan and Baluchestan provinces of Iran; investigating the antagonistic effects of actinomycetes against melon black stem or charcoal rot disease; evaluation of siderophore production by isolates in vitro; identification of potential isolate by PCR, and investigating their biocontrol efficacy against Macrophomina phaseolina in melon under greenhouse condition.

Eighty actinomycete isolates were isolated from the soil of selected different melon cultivation areas and their antifungal activity against Macrophomina phaseolina was investigated. Potential isolates were evaluated for biological activities. Moreover, the efficacy of selected actinobacteria in order to biocontrol charcoal rot disease was investigated under greenhouse condition.

Three actinomycete isolates (R1.6, R5.52 and R5.56) were revealed the highest inhibition zone size against Macrophomina phaseolina and selected for further investigations. All three isolates were able to colonize melon roots, produce extracellular enzymes and control disease in the greenhouse. The actinomycete isolate R5.56 was identified by sequence analysis of small ribosomal RNA subunit (16S rRNA) and based on the results this isolate had the highest similarity (98%) with Streptomyces species.

Biological control of plant pathogens, unlike the application of chemical pesticides, does not work quickly, but in successful cases, has more long-lasting effects compare to chemical pesticides. Biological control should be mentioned as a key component of the integrated pests and plant diseases management system approaches to minimize the environmental side effects and risks as the consequences of over usage of chemicals. This study is a prelude to further studies such as the use of these antagonists against Macrophomina phaseolina in the field, which must be done for at least three years.