Parasitism of Ramulariopsis pseudoglycines in cotton genotypes with different levels of resistance

Abstract

Ramularia Leaf Spot (RLS), caused by Ramulariopsis pseudoglycines, is one of the main diseases that affect cotton in Brazil. The disease is widely spread in Brazilian cotton producing region. The aim of this study was to identify critical stage on the leaf of cotton in which disease develops by the use of different microscopy techniques. The comparative histopathological characterization is among the study carried out in the evaluation of the disease resistant cultivars. Three cotton cultivars with different levels of resistance: IMA 5801 B2RF (highly resistant) (I), DP1746 B2RF (high susceptibility) (D), TMG44 B2RF (moderately resistant) (T) and two isolates of R. pseudoglycines (FAL F3 (A) and FAL FICC (B)) were used in this study. Microscopy techniques: Light Microscopy (LM) and Scanning Electron Microscopy (SEM) were used to characterize the interaction of R. pseudoglycines with leaf tissues of cotton genotypes with different levels of resistance to RLS. Using completely randomized block design, a total of 60 plastic pots were used in growing three cotton cultivars. Thirty (30) plastic pots of cotton seed containing 10 plants of each evaluated cultivars were used for severity test. The other 10 plant for each evaluated cultivars were used in determining the appearance of RLS using microscopy in the laboratory, totaling 30 plants. Plants with thirty days after germination, were kept in greenhouse, and inoculated with the isolates. The inoculated plants were transferred to dark growth chamber, for 24h, in order to carry out the experiment under the temperature range of 25 to 30oC, 12 hrs of light and 100% of relative humidity. Severity data were analyzed using the R program by means of the Area Under the Disease Progress Curve (AUDPC). Samples for microscopic analysis were collected at 12, 24, 48, 72 and 96 hours after inoculation (hai). Histopathological analysis showed higher phenolic compounds accumulation in cultivar IMA 5801 B2RF (highly resistant), followed by cultivars TMG44 B2RF (moderately resistant) and DP1746 B2RF (high susceptibility). In the SEM analysis, a high rate of germination and infection was observed at the beginning of the infectious process of the DP1746 B2RF cultivar (high susceptibility), while for the other cultivars with higher levels of resistance, a high rate of non-germinated conidia was observed in advanced stages of the infectious process. The highest AUDPC was observed in the DP1746 B2RF cultivar inoculated with isolate B (high susceptibility), and the other cultivars, both inoculated with isolate A and B, did not differ statistically.