Incidence of Maize Streak Disease in Maize Cultivated During Late Cropping Season in Three Agricultural Zones of Benue State, Nigeria

Maize, an important staple crop is emerging as cash crop particularly for smallholder farmers in Nigeria. This paper evaluates the frequency of maize streak disease on maize sown during late cropping season in three agricultural zones of Benue State, with three maize cultivars; Super 98, Obasuper 1 and a local cultivar at the University of Agriculture Makurdi Research Farm using standard methods. Maize plots were sampled for streak disease from July to November. Data were analysed using descriptive statistics and ANOVA with means separated using FLSD(p=0.05). Spread of maize streak was low (< 20.0%) across the agricultural zones. It was, however, significantly different (p≤0.05) among the zones with 17.2% average, although reached 19.2 % at Zone B in one occasion. Streak severity was equally significant (p≤0.05) but high (3.6%) across zones, reaching its peak, 3.7 at Zone C while zone B (3.4) was least. Disease spread among the LGAs in the State equally differed significantly (p<0.05). Gwer West (21.8%) and Otukpo (19.3%) had the highest spread. Lowest percentage spread of 12.7% and 14.0% were recorded in Konshisha and Katsina Ala, respectively. Streak severity also differed statistically (p<0.05) among the LGAs. Ado and Konshisha were highest with 3.9 and 3.7, respectively while Gwer West, which was highest for disease spread was least in severity. Disease spread in maize experimental plots was more or less comparable to spread in farmers' fields. It was low averaging 10.3%, in plots while severity of disease was high (≥3.5) in experimental plots. Keywords: Benue State, Disease spread, Late season, Maize streak, Zea mays

Dating the origins of the maize-adapted strain of maize streak virus, MSV-A

Maize streak virus (MSV), which causes maize streak disease (MSD), is one of the most serious biotic threats to African food security. Here, we use whole MSV genomes sampled over 30 years to estimate the dates of key evolutionary events in the 500 year association of MSV and maize. The substitution rates implied by our analyses agree closely with those estimated previously in controlled MSV evolution experiments, and we use them to infer the date when the maize-adapted strain, MSV-A, was generated by recombination between two grass-adapted MSV strains. Our results indicate that this recombination event occurred in the mid-1800s, ∼20 years before the first credible reports of MSD in South Africa and centuries after the introduction of maize to the continent in the early 1500s. This suggests a causal link between MSV recombination and the emergence of MSV-A as a serious pathogen of maize.

Recombination, decreased host specificity and increased mobility may have driven the emergence of maize streak virus as an agricultural pathogen

Maize streak virus (MSV; family Geminiviridae, genus Mastrevirus), the causal agent of maize streak disease, ranks amongst the most serious biological threats to food security in subSaharan Africa. Although five distinct MSV strains have been currently described, only one of these – MSV-A – causes severe disease in maize. Due primarily to their not being an obvious threat to agriculture, very little is known about the ‘grass-adapted’ MSV strains, MSV-B, -C, -D and -E. Since comparing the genetic diversities, geographical distributions and natural host ranges of MSV-A with the other MSV strains could provide valuable information on the epidemiology, evolution and emergence of MSV-A, we carried out a phylogeographical analysis of MSVs found in uncultivated indigenous African grasses. Amongst the 83 new MSV genomes presented here, we report the discovery of six new MSV strains (MSV-F to -K). The non-random recombination breakpoint distributions detectable with these and other available mastrevirus sequences partially mirror those seen in begomoviruses, implying that the forces shaping these breakpoint patterns have been largely conserved since the earliest geminivirus ancestors. We present evidence that the ancestor of all MSV-A variants was the recombinant progeny of ancestral MSV-B and MSV-G/-F variants. While it remains unknown whether recombination influenced the emergence of MSV-A in maize, our discovery that MSV-A variants may both move between and become established in different regions of Africa with greater ease, and infect more grass species than other MSV strains, goes some way towards explaining why MSV-A is such a successful maize pathogen.