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Soybean stem rot and STRATEGIES TO MANAGE IT

October 2020

Lisa Ann Rothmann, University of the Free State. Send an email to CoetzeeLA@ufs.ac.za  

Sclerotinia sclerotiorum is a yield-limiting fungal plant pathogen, present on every crop-producing continent with over 500 known susceptible host plants, including weeds. In south africa, hosts include (but are not limited to), cabbage, canola, cauliflower, drybean, hubbard squash, soybean, sunflower, pea and potato.

The diseases caused by S. sclerotiorum are characterised by the distinguishing features namely, watery soft rot (Photo 4A; symptoms), followed by white cottony mycelia (Photo 4B to 4D), a shredded appearance (Photo 4B) and ultimately melanised mycelium, known as sclerotia (Photo 1A1B and 6; signs).

The sclerotia are crucial to the lifecycle of this pathogen, they are the survival structures and with the ability to survive for up to eight years in and on the soil (Photo 1). The complexity of this pathogen is complicated due to the sclerotia affording this pathogen the opportunity to form two inoculum types, mycelia (product of myceliogenic germination; Photo 2A) and ascospores (product of carpogenic germination; Photo 2B). These germination pathways are induced under contrasting environmental conditions.

Carpogenic germination usually occurs under lower temperature ranges than that of myceliogenic germination, however, both pathways prefer high relative humidity, moisture or leaf wetness. The initiation of the stipes from sclerotia leads to the development of apothecia (Photo 3A), a mushroom-like structure, which appears much like a teacups’ saucer (Photo 3B). Ascospores, infection propagules, are forcibly discharged from apothecia when air pressure changes are observed within the canopy, widely dispersing spores. Apothecia are frequently misidentified as the common bird’s nest fungus (Photo 3B), belonging to the Nidulariaceae family. In contrast, mycelium of S. sclerotiorum is responsible for crown and stem infections nearer to the soil surface (Photo 4D).

The complexity of managing Sclerotinia diseases are extenuated as no conventional resistance exists within any of the host crops. Management strategies have thus relied on reducing the opportunity for the sclerotia to germinate and thus keep the sclerotial population limited, as well as ensuring the disease initiation risk is limited. Although, chemical control is available, in South Africa, there are limited registered active ingredients to manage diseases caused by S. sclerotiorum.

Benomyl is registered as a sunflower seed treatment. The remaining active ingredients listed are recommended for application at early bloom, ~1% to 20% flowering depending on the crop. Procymidone is registered for the application on dry bean, green bean, soybean and pea. Sclerotinia stem rot management of canola is possible with either azoxystrobin or a prothioconazole + tebuconazole combination. While leafy vegetables, such as lettuce, can be controlled with a combination of cyprodinil + fludioxonil (AVCASA, 2018).

Registered products are applied at different application frequencies in fields due to the nature of disease variation between provinces, as a result of pathogen virulence and inoculum potential, host susceptibility and environmental conduciveness. As a result of the wide host range, pathogen biology, and environmental dependence the management of Sclerotinia diseases requires an integrated approach. Planting dates, selecting cultivars less sensitive to S. sclerotiorum, crop rotations, weed management, population densities, tillage practices and biological control been integrated to manage Sclerotinia epidemics.

Publication: October 2020

Section: Pula/Imvula