Pathogenic variability and sexuality in Peronosclerospora sorghi (Weston and Uppal) Shaw, and comparative nuclear cytology of Peronosclerospora species

Abstract

Sixteen Peronosclerospora sorghi populations from seven countries were evaluated for variability in their virulence pattern on sorghum. All populations were identified as distinct vertical pathotypes based on differential sporulation on defined sorghum differentials. Cultivar-by-isolate interactions were consistent with Flor's gene-for-gene hypothesis. Sorghum cultivars IS1032, IS2473, IS3546, IS14332, IS14387, IS22227, IS22228, IS22229, IS22230, and QL-3, India were resistant to all isolates of P. sorghi tested. These cultivars are considered to be stable and durable sources of sorghum downy mildew (SDM) resistance. Most of the SDM-resistant converted cultivars from Texas remained resistant to isolates from the Americas, whereas they were susceptible to the isolates from India and Africa. P. sorghi isolates from India and Africa had a wider range of virulence compared to the isolates from the Americas. The correlation coefficient between degree of conidial sporulation on inoculated sorghum cultivars and percent SDM incidence was 0.9348. To determine the diversity in sources of resistance, sorghum cultivars that possess resistance to Texas pathotypes 1 and 3 were inoculated with conidia at seed germination. On the basis of percent SDM incidence, cultivars were grouped into resistant, heterogeneous, and susceptible classes. Sexuality in P. sorghi was evaluated by attempting to isolate mating types and by establishing the relationship between origins of gametangia. Attempts to isolate mating types failed. Light microscopy and scanning electron microscopy revealed that male and female gametangia are borne on the same vegetative hypha, i.e., that P. sorghi is homothallic. One isolate of P. sorghi (Thailand) was compared with five isolates of P. sorghi for mean number of nuclei per conidium. The mean number of nuclei (10.07) of purported P. sorghi (Thailand) differed significantly from mean number of nuclei (14.26) of the species P. sorghi. The mean of purported P. sorghi (Thailand) was significantly different from each of the other P. sorghi isolates. Three isolates each of P. sacchari and of P. philippinensis were compared for mean number of nuclei. Significant differences among isolates were evident within both species, but mean number of nuclei of species P. sacchari was not significantly different from the mean of species P. philippinensis.