R. Kahuthia-Gathu

Diversity, distribution and role of wild crucifers in major cabbage and kale growing areas of Kenya

An investigation of the diversity and distribution of wild crucifer species and their importance for cultivated crucifers was conducted during 2005 and 2006 in the highland and mid-altitude semi-arid areas of Kenya. Thirteen species of wild crucifers in nine genera were recorded: Raphanus raphanistrum, Erucastrum arabicum, Sisymbrium officinale, Crambe kilimandscharica, Capsella bursa-pastoris, Rorippa nudiuscula, Ro. micrantha, Ro. microphylla, Lepidium bonariense, Coronopus didymus, Brassica rapa, B. juncea and an unidentified Brassica species. Highland areas had significantly higher species diversity and species richness than mid-altitude semi-arid areas. Species richness, diversity and evenness varied with season and location. Raphanus raphanistrum was the dominant non-cultivated species in the highlands followed by E. arabicum, which was also present and dominant in the semi-arid study sites. Diamondback moth (DBM) was recorded from ten wild crucifer species and R. raphanistrum and E. arabicum were the preferred host plant species. Overall, four larval, one larval-pupal and one pupal parasitoid of DBM were recorded: Diadegma semiclausum, D. mollipla, Apanteles sp., Cotesia plutellae, Oomyzus sokolowskii and Brachymeria species, respectively. Diadegma semiclausum was the most dominant species on all crucifers. We conclude that wild crucifers act as alternative hosts for DBM and provide refugia for DBM parasitoids, which risk local extinction through pesticide application or competition from introduced exotic parasitoid species. The wild crucifers also act as recolonization sites for DBM parasitoids.

Seasonal incidence of Plutella xylostella (Lepidoptera: Plutellidae) and its associated natural enemies in major crucifer growing areas of Kenya

Seasonal incidences of Plutella xylostella, diamondback moth (DBM) and its associated natural enemies were investigated in two agro-ecological zones of the major crucifer growing areas of Kenya in 2005 and 2006. DBM larvae and pupae were collected from the cabbage and kale crops grown in farmers’ fields and maintained in the laboratory for the emergence of parasitoid or DBM. Four larval, one larval-pupal and one pupal parasitoid species were recorded from DBM. The parasitoids recovered were Diadegma semiclausum, D. mollipla, Itoplectis spp., Cotesia plutellae, Apanteles spp., Oomyzus sokolowskii and Brachymeria species . D. semiclausum was the most dominant species throughout with highest parasitism rates of over 70% recorded in the highlands. C. plutellae , Apanteles and Brachymeria were recovered from mid-altitude between 800 and 125 m above sea level (ASL) in the semi arid areas. Generally, parasitism was significantly higher on B. oleracea var. capitata. D. semiclausum displaced the indigenous parasitoids from B. oleracea var. capitata.

Good Orchard Maintenance and Agronomic Practices as Working Components in Management of Dieback Disease on Passion Fruit (Passiflora sp.) in Kenya.

Passion fruit is of high economic importance in Kenya. In the recent past, diseases have led to decline in fruit production. This study aimed at identifying disease management practices that could be adopted to mitigate disease impacts. Passion fruit varieties used include purple passion (Passiflora edulis Sims) and the KPF hybrid. Field maintenance practices assessed include frequency of orchard sanitation through regular weeding and pruning, pesticide application, nutrient and water provision to plants during the dry season. Agronomic factors assessed include grafting, prunning and effect of wounding in increasing plants’ susceptibility to dieback infections. Assessments were done on-farm and on station; 3 research sites were identified in the field and 1 site was set up on-station. For each of the 3 on-farm sites, a farmer’s performance in orchard maintenance was rated for all the factors on a score of 1-5 and compared to disease severity (1-5 dieback scale ) and incidence (0-100%). Experiments were set up to assess role of grafting on 4 months Original Research Article Annual Research & Review in Biology, 4(9): 1397-1405, 2014 1398 old purple passion seedlings, while effects of prunning and wounding were assessed on 9 months old passion fruit plants of both varieties. Disease establishment was observed and severity was assessed based on the 1-5 dieback scoring chart. Results showed that proper field maintenance reduced disease incidence and severity (12% and 1.6, respectively) in site 1, while poor maintenance yielded higher levels (55% and 4.0, respectively) in site 3. Agronomic practices important in transmission of dieback pathogens on the purple variety were pruning, grafting and wounding. These were found to enhance disease establishment and severity (P ≤ 0.05) on the purple variety while KPF 12 was tolerant to dieback infections. The study established that proper agronomy and field maintenance practices are important and should be integral in control of dieback disease.