Distribution and characterization of sweet potato alternaria blight isolates in Uganda
In Uganda, sweetpotato virus diseases and Alternaria are the major diseases affecting sweetpotato production. Most of the improved varieties and landraces grown for home and commercial consumption like NASPOT 1 and New Kawogo are highly susceptible to Alternaria blight caused by the fungus Alternaria. The population genetic structure of Alternaria species in Uganda is currently unknown, consequently, it is difficult to reliably determine whether popular resistant cultivars will remain resistant over a wide geographical area, or not. This information is critical in breeding for resistance to the disease. A study was carried out at Makerere University to characterize sweetpotato Alternaria isolates, and to assess their pathogenicity on selected sweetpotato cultivars in Uganda. Alternaria isolates (299) were recovered from local popular and improved cultivars from 170 locations in 17 districts of Uganda. Isolates were characterized using morphological characters, pathogenic and molecular techniques; Random amplified polymorphic DNA (RAPDs) and restricted fragment length polymorphism (RFLP). Disease severity was significantly (P< 0.05) varied between districts surveyed. Wakiso district had the highest (3.2) severity scores whereas Sironko had the lowest severity scores (1.2). A study of the 299 Alternaria isolates revealed substantial variability in growth rate, color, morphology and sporulation. Colony morphologies of isolates were compared on Potato Agar (PDA), Sweetpotato vine decoction medium (SPVDM) and Calcium carbonate (CaCo3). Mean increase in diameter in seven days varied greatly by species type ranging from 0.1 to 3.0 mm. In addition, morphological markers revealed two species of Alternaria, A. bataticola and A. alternata. Randomly amplified polymorphic DNA (RAPDs) and restricted fragment length polymorphism (RFLP) were able to differentiate the two species into two clusters with many sub groups. Pathogenicity tests of six isolates were carried out on both detached leaves and potted plants. The cultivars; Ebwanaterak (highly resistant), Magabali (moderately resistant) and NASPOT 1 (highly susceptible) were used as host differentials to determine variation in pathogenicity, aggressiveness and possible host specificity among Alternaria isolates. The three cultivars differed significantly (P < 0.01) to all A. bataticola and A. alternata infection.. The most virulent isolate was NTG 215 which was able to attack the most resistant cultivar, Ebwanaterak. However, A. alternata isolates only caused mild symptoms to all the cultivars. This proved occurrence of three different strains of A. bataticola and a single strain for A. alternata. A. alternata is not a major threat to sweetpotato production since it showed mild symptoms. Existence of 3 different races of A. bataticola as revealed by race differentials is a major threat to sweetpotato production. Breeding should either target the most virulent race from Ntungamo or could be location specific for the races.