Charcoal rot (Macrophomina phaseolina) across melon diversity: evaluating the interaction between the pathogen, plant age and environmental conditions as a step towards breeding for resistance

Macrophomina phaseolina is a soil-borne fungal pathogen that incites charcoal rot in more than 500 plant species including melon, Cucumis melo. Disease incidence and severity are affected by host genetic background, plant age, temperature, and water economy. The non-genetic variation in disease severity exhibited by infected melon plants poses a challenge to breeding for resistance. The objective of this investigation was to advance toward a fast and reliable screening methodology for identifying resistant melon germplasm, to facilitate breeding for resistance to M. phaseolina. To achieve this goal, plants of 25 melon accessions were inoculated with M. phaseolina using the toothpick method and the plants were defined as resistant or susceptible to the pathogen based on degree of disease severity. Young plants of the melon accessions were tested in a plastic greenhouse, a glass greenhouse, and a growth chamber to assess the effects of the environment, primarily temperature, on disease severity. Also tested were maturing field-grown plants as well as branches that were detached from them and inoculated in the laboratory. Differences in disease severity among accessions were most evident under the high temperatures of the glasshouse experiments conducted in mid-summer. Two accessions, Qishu Meshullash and PI 164323, were the most consistently resistant to M. phaseolina over the wide range of environmental conditions posed by the present experiments. When only the six most resistant and six most susceptible accessions were considered, moderate to high correlations (r = 0.62–0.96) in disease severity were observed between the field-grown plants and their detached branches. Results from a half-diallele crossing scheme involving six of the accessions indicated that resistance of melon to M. phaseolina has both, dominant and additive components.