~ 444 ~ Journal of Entomology and Zoology Studies 2018; 6(5): 444-447 E-ISSN: 2320-7078 P-ISSN: 2349-6800 JEZS 2018; 6(5): 444-447 © 2018 JEZS Received: 25-07-2018 Accepted: 27-08-2018 Y Sireesha Assistant Professor, Department of Plant Pathology, College of Horticulture, Anantharajupeta, Andhra Pradesh, India R Velazhahan Professor, Department of Crop Sciences, College of Agricultural and Marine Sciences, Sultan Qaboos University, Al-Khodh, Muscat, Sultanate of Oman Correspondence Y Sireesha Assistant Professor, Department of Plant Pathology, College of Horticulture, Anantharajupeta, Andhra Pradesh, India Assay of cross-infectivity between isolates of Peronosclerospora sorghi causing downy mildew on maize and sorghum Y Sireesha and R Velazhahan Abstract Downy mildew, caused by Peronosclerospora sorghi (Weston & Uppal) Shaw, is an economically important disease of maize (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench] in India. The symptoms of downy mildew on sorghum and maize are difficult to distinguish from each other using morphologic criteria. Hence cross infectivity assay was conducted to monitor the development of symptoms in maize and sorghum after reciprocal inoculation. When maize seedlings were inoculated with P. sorghi isolates from maize and sorghum, typical symptoms of downy mildew appeared 5 days after inoculation. Sorghum isolate induced more disease (82%) on maize compared to maize isolate (71%). When sorghum seedlings were inoculated with P. sorghi isolate from maize, typical symptoms of downy mildew were produced. In contrast, sorghum seedlings when inoculated with maize isolate did not produce any symptom of the disease indicating pathogenic variability among the isolates of P. sorghi from sorghum and maize. Keywords: Cross inoculation, P. sorghi, downy mildew, maize, sorghum Introduction Diseases are serious constraints to maize production worldwide. Maize diseases can cause harvest losses, affect the quality of the harvested crop and cause storage losses. Among the various diseases, downy mildew, caused by Peronosclerospora sorghi (Weston & Uppal) Shaw, is a major limiting factor to maize cultivation worldwide [7] . P. sorghi can cause significant yield losses under favourable environmental conditions and yield losses can reach 50-100% in susceptible cultivars [10] . Although this disease can occur at any stage of maize development from seedling to harvest, the fungus primarily infects maize soon after the emergence of seedling, until one month after planting [10] . The leaves of affected plants show chlorotic streaks on the upper surfaces and white, cottony growth consisting of sporangiophores and sporangia on the corresponding lower surfaces. The leaves later become necrotic and finally dry. The tassels are malformed as small, green leaf-like structures, called “crazy top” that makes the plant unproductive. Epidemic development of the disease within a season is mainly due to airborne conidia and oospores are involved in the long distance movement through seed and soil [1] . The primary source of inoculum is infected seed and airborne spores play an important role in dispersal of the pathogen. Besides maize, P. sorghi has been reported to infect sorghum [Sorghum bicolor (L.) Moench], forage sorghum (Sorghum vulgare Pers.) and tanglehead (Heteropogon contortus L.) [6] . Although P. sorghi can infect both maize and sorghum, it typically does not complete sexual reproduction on maize; hence no oospores are formed [16] . Olanya and Fajemisin (1993) reported existence of two pathotypes of P. sorghi, one capable of infecting both maize and sorghum and the other specific to maize. The P. sorghi infecting sorghum and maize are difficult to distinguish from each other morphologically [2] . However, the evidence for molecular variability among P. sorghi isolates have been reported from many parts of the world, including India [9, 20] , Africa [3] , Indonesia [10] and the United States [16] . Such variabilities among pathogen populations are more likely to overcome genetic resistance [12] . Although P. sorghi can infect both maize and sorghum, it typically does not complete sexual reproduction on maize, and hence no oospores are formed [16] . Hence cross infectivity assay was conducted to monitor the disease development of symptoms in maize and sorghum after reciprocal inoculation and to know the primary infection in maize.