Screening of butternut (Cucurbita moschata) cultivars for resistance to Black rot caused by Stagonosporopsis spp.

Abstract

Gummy stem blight, black rot, and brown etch, are all caused by Stagonosporopsis cucurbitacearum, Stagonosporopsis citrulli and Stagonosporopsis caricae, a group of fungi known as a species complex. Given the recent risk associated with fungicide resistance development to this group of pathogens, alternative control measures such as genetically resistant cultivars, need to be considered. This study aimed to establish whether or not various levels of resistance to black rot exist among selected South African Cucurbita moschata cultivars. Twenty-four S. cucurbitacearum and five S. caricae isolates were obtained from symptomatic plant material. The identity of the fungal species was confirmed using a set of three conventional polymerase chain reaction (PCR) primer pairs, and pathogenicity of each isolate tested. There was a significant difference in levels of virulence among the isolates, with two of the most virulent isolates, one of each (S. cucurbitacearum and S. caricae) were selected for use in the “resistance-screening” trial. The resistance-screening trials consisted of inoculating fifteen C. moschata cultivars at one month and six months post-harvest. The resistance-screening trial for fruits with a post-harvest age of six months was repeated, but with ten C. moschata cultivars. Each fruit was inoculated five times with toothpicks: twice with S. cucurbitacearum, twice with S. caricae and once with a negative control toothpick. Black rot symptoms were evaluated by measuring the size of the exocarp lesions, as well as the sizes of affected mesocarp tissue. Both mesocarp and exocarp lesion evaluations indicated that cultivar ‘T’ was most resistant to black rot disease across all three growing seasons, while cultivars ‘909’, ‘Q’ and ‘50’ were most susceptible. Cultivar ‘18’, although resistant to exocarp infection, mesocarp tissue was most susceptible to black rot. Infected tissue was excised from the diseased lesions and pathogen DNA quantified for both exocarp and mesocarp tissue, using quantitative polymerase chain reaction (qPCR) assays. The results correlated with phenotypic evaluations, in which cultivar ‘T’ tissue contained the least amount of pathogen DNA, and cultivars ‘909’, ‘50’ and ‘Q’, the most. Butternut cultivars responded similarly to both species of the pathogen, however, S. caricae was more virulent than S. cucurbitacearum. This study confirmed variability with regard to levels of resistance to black rot among some South African C. moschata varieties. Stagonosporopsis caricae was also demonstrated to be more virulent than S. cucurbitacearum. As fruits aged post-harvest, exocarp and mesocarp tissue increased in susceptibility to symptom development. The quantity of pathogen DNA, however, was not influenced by fruits post-harvest age, therefore considerable effect of fruit ripeness on actual black rot development, affects phenotypic symptoms only.