Distribution, severity and molecular variations in angular leaf spot of beans caused by phaeoisariopsis griseola in selected areas of Zambia.
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Fungal angular leaf spot disease of the common beans is caused by the pathogen Phaeoisariopsis griseola (Sacc.) Ferarris synonym, Pseudocercospora griseola. The pathogen is estimated to cause about 60-100% yield loss. A survey was carried out to assess the prevalence, severity and incidecnce of angular leaf spot disease and it was established that angular leaf spot was prevalent in all the 41 fields surveyed in the nine districts. The highest percentage severity index was recorded in Mpika (28%) and the lowest was in Kaputa (2.2%). The highest average severity was in Muchinga province. However, ANOVA showed that there was a significant difference in severity among provinces with F=3.36, p=0.045. The incidence was high with eight fields having incidence greater or equal to 80%. There was no significant difference in incidence between provinces with F=2.17, p=0.128. Most of the farmers in the surveyed fields used seed which was recycled and also bought planting seed from the market. Nevertheless, a multiple regression revealed that source of seed and cropping system did not contribute a significant amount of change in disease incidence with R2 = 0.076. The main cropping system which was practiced by the farmers was monocropping where as very few farmers practiced intercropping but there was no significant difference in disease incidence in monocropped and intercropped fields t(39)=1.72, p=0.93. Symptomatic bean leaf samples were also collected from each field and were cultured on potato dextrose agar, V8- juice agar, and water agar. The resulting condia were identified using published literature. Polymerase chain reaction of the P. griseola genomic DNA which was run using selected ISSR markers (UBC 889,888 and 809) revealed high genetic diversity among the 91 isolates. The genetic diversity was assessed using similarity indices which was in the range of 0 – 1, Shannons H =3.16 and the Simpsons 1_D = 0.93 thus showing that indeed there is diversity among the isolates. The concatenated tree dendrogram showed that isolates were clustering according to districts and in some subgroups there was a mixture of isolates from different districts. The results also revealed variation of isolates within the districts as well. This high genetic diversity implies that the pathogen can easily overcome existing resistance in the bean plants thereby leading to higher infections. This calls for regular sceening of existing and new varieites for resistance to angular leaf spot disease. The high genetic variability also means that new pathotypes will arise over time thus creating the need to study the pathogen further using differential cultivars to determine which pathotypes are present in Zambia.
The University of Zambia