Characterization of Fungal Leaf Spot Disease and Varietal Susceptibility in Sweet Potato (Ipomoea Batatas L.) Grown in Parts of Western Kenya

Increased human population has led to increased vulnerabilities to hunger and reduced sources of affordable food. Sweet potato (Ipomoea batatas L.) is a root vegetable with large, starchy, tuberous roots, consumed worldwide. It is a potential food security crop, with high nutritional and economic value. Production of sweet potatoes in Kenya is low due to disease constraints such as sweet potato leaf spot (SPLS). Fungal SPLS results in reduced tuber yield, since the pathogen attacks photosynthetic leaves, causing premature defoliation and senescence. Information on disease incidence, pathogen genetic variability and susceptibility of some locally cultivated varieties in western Kenya is not fully documented. A characterization and varietal susceptibility study of SPLS, presents an opportunity for increased production, improved food security and enhanced income. This study evaluated SPLS disease incidence, characterized the causative pathogens, assessed susceptibility of locally grown varieties to the infection and evaluated farmers’ indigenous knowledge to sweet potato diseases. Infected leaf samples were obtained from farmers’ fields in Kabondo Kasipul and Rangwe Sub-Counties in Homa Bay County, and in Kajulu in Kisumu East Sub-County of Kisumu County where active cultivation of sweet potatoes is done. A multi-stage sampling design was used to identify exact sampling points at farmers’ fields based on acreage of sweet potato grown. At each site, disease incidence and severity was evaluated in quadrats thrown on prescribed scales within the habitable space of diseased plants. Diseased leaf samples were collected and transferred to the Botany laboratory at Jaramogi Oginga Odinga University of Science and Technology, where pathogen isolation and characterization experiments were arranged in Completely Randomized Design with three replicates. Controlled pot-experiments were conducted in a green house to test the susceptibility of 1 farmer-held and 2 improved sweet potato varieties to one isolated SPLS pathogen (NF4 - Accession No. OK560339.1). The experiments were arranged in Randomized Complete Block Design with three replicates. A structured questionnaire was administered to 66 farmers to survey farmer’s indigenous and perception of the disease. Data on disease incidence, severity and susceptibility were transformed into percentages and subjected to Analysis of Variance using General Linear Model and means separated using Least Significant Difference (LSD0.05) on Scientific Analysis System (SAS) version 9.4. Morphological data was numerically transformed on MS Excel and hierarchical relationships determined using a dendogram. Genomic DNA was extracted from pure samples, and the Internal Transcribed Spacer (ITS) gene amplified by Polymerase Chain Reaction (PCR), followed by sequencing at Macrogen, UK. Molecular data were assembled and trimmed on CLC Main Workbench (CLC Bio, Version 6.8.3). Assembled sequences were transferred to MEGA Version 6 and aligned using CLUSTAL W. Data on farmer indigenous knowledge was analyzed using Chi-Square (χ2 ) test on SPSS version 25. Results on disease incidence ranged from 11% to 30.38% at Kakelo and Kamollo villages respectively, while SPLS severity was significantly (p