Pathogenicity studies on strawberry to identify sources of resistance against Macrophomina phaseolina

Abstract

Charcoal rot, caused by soil-borne fungal pathogen Macrophomina phaseolina (Tassi) Goidanich, is an important disease that affects strawberry cultivation in Australia. The pathogen has also been reported to cause significant plant losses worldwide and its emergence has been associated with the loss of the soil fumigant methyl bromide. Current fumigants are not able to eradicate the pathogen as effectively as methyl bromide. Management of the pathogen is also difficult due to its wide host range and ability to persist in the soil. The development of charcoal rot resistant genotypes is a major focus of the Australian Strawberry Breeding Program (ASBP). There are, however, several key gaps in the knowledge of charcoal rot of strawberry in Australia in order to breed for resistanceTo address the lack of knowledge on host-pathogen interactions, a series of glasshouse experiments were conducted during 2015-2018 at the Maroochy Research Facility in Nambour, Australia. The first of the objectives was to develop an efficient and reliable inoculation method to be used for future disease screening. Two techniques were evaluated on two strawberry genotypes: inserting a M. phaseolina colonised-toothpick into the plant and drenching the potting media with a suspension of microsclerotia. A second objective was to identify genotypes with sources of resistance to M. phaseolina, where current and historical strawberry genotypes were screened. The third objective was to characterise M. phaseolina isolates for diversity and pathogenicity to strawberry, with the aggressiveness of thirty Australian M. phaseolina isolates investigated. Lastly, a genotype by isolate study was conducted to determine if there was a pathogenicity relationship between plant genotype and M. phaseolina isolate.M. phaseolina colonised-toothpicks were able to cause charcoal rot symptoms on two genotypes, and the likelihood of mortality at any given time was the same for both genotypes using this method. On the other hand, drenching plants with a M. phaseolina microsclerotia suspension found that one cultivar had a higher likelihood of mortality, suggesting inoculation by this method may better differentiate susceptibility between genotypes. For this reason, drenching potting media with a suspension of microsclerotia was the technique selected for subsequent pathogenicity assays for this study and for future breeding line screening.Studies on the strawberry genotype response to M. phaseolina showed the genotypes tested had varying degrees of resistance. Historical cultivars ‘Earlibrite’, ‘Kabarla’ and ‘Phenomenal’ were shown to be the most resistant to M. phaseolina. The cultivars ‘Camarosa’, ‘Albion’, Strawberry Festival’, ‘Rubygem’, and ‘Florida Radiance’ were the least resistant.M. phaseolina isolates from Australia were non-specific as all isolates tested in this study were able to cause charcoal rot symptoms on strawberry. There were significant differences, however, in the pathogenicity of the thirty isolates of M. phaseolina assessed, which categorised into two different groups based on the aggressiveness of the isolates to strawberry. With the exception of one isolate, strawberry isolates were more aggressive to strawberry, as were isolates from sorghum grown as cover-crop in a cultivated strawberry field, compared with isolates from sorghum, chickpea, mung bean, peanut and watermelon. The result suggests isolates that were more adapted to strawberry showed host- specialisation. The study also confirmed, based on molecular analysis, that a representative number of isolates were M. phaseolina, and were distinct from a closely relates species, M. pseudophaseolina.A cultivar by isolate study found that genotype and isolate effects were significant. This reinforced the earlier findings that there are sources of resistance in the ASBP strawberry genotype collection, and that M. phaseolina isolates that originated from strawberry had host- specialisation (i.e. were more aggressive) towards strawberry plants. There was no significant cultivar by isolate effect, suggesting that a single isolate could be used for future resistance screening.The knowledge gained from this study has immediate beneficial implications for the strawberry industry in Australia. The identification of genotypes with sources of resistance to M. phaseolina will guide selections by the ASBP to produce strawberry cultivars with resistance to charcoal rot. Large numbers of strawberry genotypes developed by the ASBP can now be evaluated for charcoal rot resistance/tolerance using a reliable and effective screening technique that incorporates susceptible and tolerant genotypes and M. phaseolina isolates with known pathogenicity to strawberry. Ultimately, the use of resistant cultivars is a more effective and sustainable alternative to current soil fumigants and in the management of charcoal rot of strawberry.