Plant age in relation to Cucumber mosaic virus disease severity on okra (Abelmoschus esculentus L. Moench) in southern Guinea savanna agroecology of Nigeria

Abstract

The severity of Cucumber mosaic virus (CMV; genus Cucumovirus; family Bromoviridae) infection on okra (Abelmoschus esculentus) plants infected at different growth stages was investigated in Minna (6.44oE, 9.51oN; 220 m above sea level), Southern Guinea Savanna of Nigeria. Two field experiments were conducted simultaneously using Randomised Complete Block Design (RCBD) with four replications. Seeds of the okra cultivar “Ikeregi” which is commonly grown in Minna were sown in the third week of May, 2014 on 5-m long ridges at inter- and intra-row spacing of 75 cm × 30 cm. Treatments consisted of uninoculated plants (control), CMV-inoculated at 7 days after sowing (DAS), 14 DAS and 21 DAS. Plants were assessed for disease incidence, disease severity, growth and yield parameters. Data were subjected to analysis of variance (ANOVA). Significance of the F- test was determined at 5 % probability level and means were separated using the Least Significant Difference (LSD). One hundred percent CMV infection was observed on all the plants regardless of time of inoculation. However, disease severity varied with time of infection. At 8 weeks after inoculation (WAI), disease severity was significantly (p<0.05) highest in the plants inoculated at 7 DAS (mean symptom score = 4.0), followed by those infected at 14 DAS (mean symptom score = 2.0), whereas the lowest value was observed in those infected at 21 DAS (mean symptom score = 1.0). Number of leaves per plant was significantly (p<0.05) highest in uninoculated plants (22 leaves per plant), whereas infection at 7 DAS resulted in the lowest number of leaves per plant (12 leaves/plant). The plants infected at 14 DAS had a mean of 16 leaves per plant while those inoculated with CMV at 21 DAS produced a mean of 21 leaves per plant, which was not significantly(p>0.05) different from the mean observed in uninoculated plants. The fruit weight per plant of uninoculated plants (58.5 g) was not significantly different from those infected at 21 DAS (57.5 g). Conversely, infection at 7 DAS and 14 DAS resulted in fruit weight of 23.5 and 36.5 g, respectively. The present study showed that protecting plants for the first 21 DAS could significantly reduce CMV pathogenicity in okra fields. Therefore, okra farmers should implement adequate measures so as to reduce the effect of viruses infecting okra at early stages of growth. However, integrated pest management strategy such as cultivation of tolerant varieties and use of close spacing could be incorporated as control strategies