Fusarium verticillioides infection and fumonisin synthesis as affected by maize plant stressors
Fusarium verticillioides is an important ear rot pathogen of maize that can lead to economic losses due to yield and grain quality reduction. Symptoms vary depending upon genotype, environment and disease severity. F. verticillioides can produce fumonisin B1, B2 and B3 that can cause mycotoxicoses in animals and are also statistically linked with oesophageal cancer in humans. It has been shown that abiotic factors such as substrate, temperature and water activity can have a profound effect on fumonisin synthesis. The aim of this study is to elucidate the potential effect of plant density on F. verticillioides infection and fumonisin production in maize grain. Plant density field trials (2011-2014) with a progressive decline of soil nitrogen were planted at the experimental farm of the ARC-GCI in Potchefstroom. Plant density treatments of 10 000, 20 000, 30 000, 40 000 and 50 000 plants.ha-1 were planted using CRN3505 and PAN6P-110 in a completely randomized block design, replicated three times. As nitrogen and plant density appeared to be obscuring the effects of each other, a separate plant density field trial with adequate nitrogen was planted (2015) in Potchefstroom using 8 cultivars in a split plot design, replicated three times. Naturally infected ears were harvested at 12% moisture, threshed and grain milled and subjected to qPCR (F. verticillioides target DNA) and HPLC (fumonisin levels). During the 2011/2012 and 2012/2013 seasons, chlorophyll fluorescence parameters were measured at different days after plant (DAP) to determine plant vitality at different plant densities. Leaf material from experimental plots were sampled and analysed by the Eco-Analytica laboratory of the North-West University for total nitrogen (N), carbon (C) and sulphur (S). Three pathogenesis-related proteins, chitinase, peroxidase and β-1,3-glucanase were measured at different stages of plant development to determine their role during fungal infection and fumonisin production. This study showed that under nitrogen poor conditions, cultivar choice, environment and low plant densities could lead to elevated fungal infection and fumonisin production in maize grain, placing subsistence and small scale farmers at risk. In farming systems with adequate soil nitrogen, as plant density increase, grain moisture decrease and target DNA and fumonisins increase. Applications of LAN can influence target DNA in maize grain. Only trace amounts of fumonisins were quantified and the effect of LAN is inconclusive at this stage. This study further demonstrated an increase in the available leaf nutrients (N, S and C) as well as PR proteins (chitinase and β-1,3-glucanase) to correlate with a decrease in fumonisin levels. The increase of PR proteins during critical nfection stages of the maize plant (silk and milk) is a significant finding, as maize ears are susceptible to fungal infection and fumonisin production can occur as soon as fungal infection commenced. It was unexpected though, that available leaf nutrients as well as PR proteins did not affect fungal infection, but fumonisin levels. Currently, an integrated approach is taken to manage fungal infection and subsequent fumonisin production in maize grain. Even though fungal infection can be managed, fumonisin production can be unpredictable due to genotype, environment and substrate. Chitinase and β-1,3-glucanase response to fumonisins in this study can be used in breeding programmes to improve resistance to specifically fumonisin production in maize grain. This study re-iterated the importance of appropriate management practices such as obtaining environmentally adapted seed, applying fertilizers and using the correct planting methods to improve maize yields but also manage the mycotoxin threat to end users. This study also contribute to a better understanding of maize plant defence mechanisms and the aspects of maize physiological processes and nutritional values can effectively contribute to improved management strategies of F. verticillioides fungal infection and contamination by fumonisins.