Detection of sugarcane mosaic virus and symptom expression of maize genotypes to infection by the virus in Kenya.

Abstract

Emergence of maize lethal necrotic (MLN) disease, in eastern Africa has threatened maize production in the region. The disease is caused by mixed infection of maize by Maize chlorotic mottle virus (MCMV) and a potyvirus, most frequently the Sugarcane mosaic virus (SCMV). As a major component in the MLN disease complex, the importance of SCMV in maize production in Kenya has been increasing. Since it was first reported in Kenya in 2012, MLN disease has been persistent due to a favorable environment for survival and spread by insect vectors. Similarly, continuous maize cropping in certain regions and widespread cultivation of susceptible maize germplasm that has not been screened for reaction to SCMV has maintained its persistence. The purpose of this study was to contribute to the management of MLN disease in Kenya through better understanding of SCMV diagnostics and response of two farmer-preferred maize varieties (H614D and DUMA 43) to infection by the viruses causing the disease. Identification of SCMV has been limited due to the apparent vast dissimilarity among the virus as well as its synergy with MCMV. Detection studies of Sugarcane mosaic virus in synergism with Maize chlorotic mottle virus were carried out by developing and validating PCR assays in MLN disease complex. A field survey was carried out in major maize growing regions of Kenya and leaf samples showing MLN disease symptoms collected. The virus was detected from the leaf samples using enzyme-linked immunosorbent assay (ELISA) and seropositive samples selected to ascertain the identity of SCMV. Polymerase Chain Reaction (PCR) was carried out to further detect and affirm the presence of SCMV in the collected leaf samples. Primers from literature were tested for their reliability in the detection of SCMV. New assays were developed to broaden the scope of SCMV detection. Primer assays from literature were relatively reliable while the developed assay produced the expected band sizes but had different sensitivity. This study also reports that there are two groups of SCMV based on ELISA and PCR detection. The outcomes of this study could be used in the enforcement of quarantine measures that could prevent further spread of the disease across the maize growing regions which do not have the disease. In order to determine the vulnerability period of maize to SCMV, two farmer-preferred maize cultivars H614D and DUMA 43 were inoculated with SCMV at different growth stages (V4, V8 and V12) to evaluate their reaction to infection by SCMV. Symptoms were observed over a period of 35 days at intervals of seven days. The maize plants had varied reactions depending on the genotype and growth stage of infection. The highest area under disease progress curve (AUDPC) was 129.5 and 128 in DUMA 43 and H614D inoculated at V4 growth stage and lowest when inoculated at V12 growth stage with AUDPC mean of 89.2 and 83.1 in the two cultivars, respectively. The V12 growth stage exhibited a rather late reaction to infection in regard to the severity of visual symptoms compared to the other growth stages of which they exhibited symptoms within the first week post inoculation, indicating resistance mechanisms through delayed symptom development. To investigate the effect of consecutive infection on symptom development, two susceptible maize hybrids were each infected with either MCMV or SCMV first and later inoculated with the second virus in the greenhouse. Maize plants pre-inoculated with SCMV followed by MCMV to achieve synergism were observed to have late MLN disease symptoms expressions which were noteworthy. The findings of this study strengthen the possibility that older plants are less vulnerable to SCMV, indicating that management practices aimed at protecting young maize plants at the susceptible growth stages can minimize infection and yield loss due to SCMV. Infection in nature occurs independently rather than concurrently. The results indicated that severity of symptoms in synergy might be as a result of the sequence of infection in maize plants by MLN disease causal agents. SCMV resistance could be incorporated in breeding programs as a necessity in the development of new hybrids. The findings of this study will add to the knowledge gaps in the advancement of an operative management approaches that are ecologically sustainable. There is need for frequent surveys to monitor movement and emergence of new viruses to tailor diagnostic tools to capture putative new viruses and synergisms that could be affecting maize production.