Rothmann, L.A., Meiring, M.C. and McLaren, N.W.
(University of the Free State, Plant Sciences: Plant Pathology)
Written for the Oilseed Focus Magazine June 2019
Sclerotinia sclerotiorum has an extensive host range of more than 500 plant species, including sunflower, soybean and canola. These crops play an important role in our South African economy as protein and oil seed crops. Some vegetables may serve as alternative hosts which include (but are not limited to) cabbage, potatoes, squash, carrots and tomatoes. Many common South African weeds are also susceptible to infection and can harbour the pathogen within these crop production systems. Seedling wilt may occur, although Sclerotinia head and stem rot frequently develops at flowering and pod or seed filling stages. Brown water-soaked lesions are initial symptoms that become covered with white cotton-like mycelium on sunflower heads and soybean pods as well as in and on the stems of both sunflower and soybean. The white mycelium on the face of sunflowers eventually develops into a net of black sclerotia. As the disease matures, a shredded appearance, with sclerotia between plant fibres, can be observed, particularly in sunflower. This fungus can also infect the subterranean crown and forms sclerotia within the lower stem of sunflower and soybean. The symptoms associated with a disease are a host response to the pathogen, whereas the signs are a physical reaction of the pathogen. These signs may also be seen morphologically in the laboratory. In the case of Sclerotinia diseases, mycelium (Figure 1a; mass of fungal hyphae) and sclerotia (Figure 1b) are the primary signs of the pathogen’s presence. Sclerotia, melanised masses of hyphae, are key to the life cycle of this fungus as they are the primary survival structure. Infection by S. sclerotiorum can occur through two means of germination to produce primary inoculum, namely myceliogenic and carpogenic germination. Carpogenic germination results in the formation of an initial stipe (Figure 1c) followed by the apothecia, mushroom-like structures (Figure 2a). The birds nest fungus (Figure 2b) is commonly misidentified as apothecia. Apothecia release ascospores into the air under high relative humidity and changes in air pressure, favouring long distance dispersal and infection. S. sclerotiorum has the ability to remain dormant in the form of mycelium in infected plant residues when environmental conditions are unfavourable for germination and infection. Sclerotinia sclerotiorum is highly dependent on its’ environment, both weather and agronomic conditions, for disease initiation, development and survival. Cool, wet conditions favour disease development. The disease is more prevalent in fields were crops have a dense canopy and air circulation is limited and as a result, create a favourable micro-climate for disease development. This is directly related to the population density and row spacing selected at planting, as well as the selection of cultivars prior to planting. Cultivars vary in their physiological structure, i.e. determinate or indeterminate, with the latter tending to have a denser canopy. This pathogen has a complex life cycle and interaction with its hosts and environment, which makes the management of this disease intricate. Currently, in a worldwide context, there are no commercially available resistant sunflower or soybean cultivars. However, the manner in which soybean and sunflower cultivars differ in their response to the pathogen under disease favourable conditions enable the selection of more “tolerant” cultivars that can reduce the risk of infection and yield losses. Mrs. Marlese Meiring (Plant Pathology, University of the Free State) has conducted field trials to elucidate the response of soybean and sunflower cultivars to disease potential, using regression analysis (sensu McLaren and Craven, 2008). This analysis determines the type of response as well as the relationship between Sclerotinia head and stem rot within a cultivar and changing disease potentials. Disease potential is defined as the expected mean disease incidence in a genetically diverse population under a specific set of environmental conditions. Three response types can be observed between Sclerotinia incidence in a cultivar and Sclerotinia potential, (A) cultivar tolerant to increasing disease potential; (B) cultivar intolerance to increasing disease potential and (C) cultivar having linear relationship with increasing disease potential (Figure 3). This regression methodology can be an effective and accurate tool to quantify the response of cultivars to different disease potentials and subsequently promote the selection process of cultivars for a specific disease potential. Further seasons will be added to this study to quantity the response of the cultivars across multiple localities. There are a limited number of registered fungicides in South Africa. Currently, benomyl and procymidone are used on peas and sunflower. The latter active ingredient is registered for use on soybean. Furthermore, the exorbitant cost of the chemicals and their application as well as the potential requirement for multiple sprays is an economic risk for producers. Disease forecast models serve as an early warning system that assist producers in optimising the timing of fungicide applications, and ensure optimal efficiency and healthy economic decisions. These risk assessments have been successful internationally for canola and soybean. The systems range from simple check lists to more advance mathematical modelling which is ultimately visualised as a risk percentage or proportion, followed by a recommendation to apply (as well as application timing) or withhold fungicide applications. In Europe, canola field experiments conducted between 1981 and 2004 indicated that fungicide sprays were only 27-33% cost-effective against Sclerotinia stem rot (Koch et al., 2007). Professor Neal McLaren and Mrs Lisa Rothmann (Plant Pathology, UFS) are currently in the initial stages of developing a risk analysis model to assist producers in identifying the risk for Sclerotinia disease development at critical growth stages during the cropping season. Ten-year disease and weather data were bulked to identify critical infection windows and estimate the length of infection and colonization periods. Mrs Rothmann is conducting further exploration and optimisation of data collected for the model development in collaboration with Professor Emerson Del Ponte, from the Federal University of Viçosa, in Brazil. Mrs Rothmann will also spend some time with Professor Del Ponte and his students during June of this year. Furthermore, in order to ensure that all regions of sunflower and soybean production are covered and correct estimations of risk are distributed, Mrs Rothmann has requested producer involvement with surveys which they are currently conducting. These will be available shortly on the South African Sclerotinia Research Network website. The change in agronomic decisions, environmental weather conditions, management practices and the susceptibility of germplasm has led to an increase in the importance of Sclerotinia diseases worldwide. The greater the prevalence and severity of the disease, the lower the yield and the greater the inoculum build up. In the 2017/18 season, sunflower and soybean epidemics causing up to 80% yield losses were reported in the Eastern Free State. During 2014, in South Africa, the effects of Sclerotinia stem rot of canola gained more attention due to the greater prevalence of the disease during the season compared to previous years. Although extensive literature is available regarding Sclerotinia diseases, this disease remains problematic in economically important crops, and the pathogens behaviour locally needs to be clarified. This investigation is a priority of the South African National Sclerotinia Research Network. The spread of S. sclerotiorum into critical South African crop production areas and its associated yield losses, highlights the importance of identifying and deploying effective management measures to safeguard agricultural land against the initial incursion of this pathogen. For more information please contact Lisa Rothmann at coetzeeLA@ufs.ac.za and follow the South African Sclerotinia Research Network on Facebook, Instagram and Twitter. A website will be launching soon.