Cashew leaf and nut blight disease outbreaks under unimodal rainfall pattern in Tanzania

Abstract

Cashew (Anacardium occidentale L.) which is an important cash crop in Tanzania, is susceptible to various abiotic and biotic stresses. Cashew leaf and nut blight disease caused by Cryptosporiopsis spp. is a devastating disease of cashew crop resulting in a significant reduction of nut production. We conducted a study to assess the association between weather variables, particularly rainfall and temperature, on cashew leaf and nut blight disease (CLNBD) outbreaks and establish a minimum wetness duration period for initiation and development of the disease. Poisson regression analysis indicated a significant association between rainfall and temperature with CLNBD incidence (df = 3, Chi Pr < 0.001). Rainfall was positively associated with CLNBD and a unit increase of 1 mm in rainfall caused a 2 percent increase in the percentage of disease incidence (e = + 0.01995, p < 0.011). However, temperature was negatively associated with disease incidence and a unit increase of 1 °C in temperature caused a 21.56% decline in disease incidence (e = –0.2428, p < 0 .001). Blight incidence was significantly higher (p < 0.001) during the rainy season (range: 11.7 to 36.6%) compared to the dry season (1.4 to 11.5%). Cashew trees in Mtwara District recorded higher blight disease incidence as compared to the rest of the studied areas. This study also established that symptoms for blight disease on cashew leaves appear 8 h post exposure to continuous wet periods. Thus, it appears that for the blight pathogen to initiate and complete its infection processes, a continuous wet period of at least 8 h is required. Findings from this study, particularly the association of rainfall and temperature on disease epidemics, can be used as tools for forewarning incidences of CLNBD. This would help timely initiation of appropriate management strategies.