Reaction of ipomoea batatas (l) lam lines to alternaria leaf and stem blight and effects of soil ph on disease severity

Abstract

This study was carried out to determine the causal agent of blight disease on leaf petiole and stem of sweetpotato in different agro-ecological zones in Kenya, investigate its optimal in-vitro growth and sporulation conditions and determine the importance of the pathogen in disease development under different soil pH levels as well as field conditions. A total of 5 different zones were visited. Diseased leaf and vine tissues were collected and pathogen isolated. The Kabete, Kakamega, Busia (Alupe), and Kabondo isolates were identified as Alternaria bataticola while the Kisii isolate was identified as Alternaria alternata. The Kabete isolate was more virulent than other isolates and other species also. A. bataticola grew rapidly and sporulated abundantly on host based media; sweetpotato vine decoction media (SPVDM) and sweetpotato leaf decoction media (SPLDM) as opposed to potato dextrose agar (PDA) and malt extract agar (MEA). Significant mycelial growth occurred under alkaline pH ranges {pH 7 (71.67mm), 8 (71.81mm) and 9 (71.90mm)} while abundant sporulation occurred at acidic pH levels with optimal level at pH 5.6 (2.03 x 106 conidia/ml) on SPVDM. But, with prolonged incubation period, there was decline in sporulation at all pH levels tested. 12hrs continuous light alternating with 12hrs continuous darkness (12hrsL/12hrsD) sustained the highest sporulation while 24hrs continuous light (24hrsL) enhanced a better mycelial growth and 24hrs continuous darkness (24hrsD) sustained the lowest mycelial growth and supported less sporulation. Alternaria bataticola grew optimally at temperatures between 26°C and 28°C. Yields of conidia obtained from cultures washed by 10ml of water ranged from 1.83 x 106 conidia/ml at 28°C to 0.33 x 106 conidia/ml at 34°C. The ability of the fungus to sporulate declined at higher temperatures. Pathogenicity and virulence of different isolates of A. bataticola and A. alternata from diseased samples were investigated under greenhouse conditions. Within 4 - 6 days following artificial inoculation, and depending on sweetpotato cultivar, the fungus produced characteristic symptoms typical of Alternaria leaf petiole and stem blight. All the isolates were pathogenic and differences in virulence depended on isolate source and conidial concentration. The Kabete isolate was more virulent than Kakamega, Kabondo and Alupe isolates, whose virulence did not differ significantly from each other (P<0.05). The effect of soil pH on disease severity in some cultivars of sweetpotato was also evaluated under greenhouse conditions. There was no significant difference among the levels of soil pH tested on disease severity (P<0.05). The mean disease severity for soil pH 4.4, 4.6 and 6.7 were same (3.42) and for soil pH 7.6 was 3.25. There were significant differences in disease severity among the cultivars evaluated. Alternaria leaf and stem blight caused by Alternaria bataticola was monitored in field plots of sweetpotato accessions selected from the CIP gerrnplasm collection field. Disease severity was measured as percent infected area and used to compute area under disease progress curves (AUDPC). AUDPC's revealed distinct differences in disease infection among the accessions evaluated (p=0.05). Cultivars Viola (440046) and Yanshu 1 (440024) were more susceptible to the pathogen than other entries tested. The lowest disease levels were observed on cultivar Jayalo in both seasons.