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Chemical control of Sclerotinia diseases:

A fungicide-resistance approach

By Lisa Rothmann and Marlese Meiring

ungicides, the agrochemicals variation between provinces, as a The control of canola stem rot in
controlling fungal pathogens, result of pathogen virulence and China is managed with the application
have been contributing relief to inoculum potential, host susceptibility of dimethachlon (a dicarboximide, the
crop management systems for and environmental conduciveness. same fungicide group as procymidone).
F centuries, protecting yields and Chinese isolates collected and evaluated
assisting in providing uncompromised Pathogen sensitivity indicated resistance or insensitivity
quality. Over time, many new chemistries Fungi that are successfully controlled to the fungicide applications in the
with novel structures and systemic activity by fungicides are known as ‘sensitive’ to laboratory and in field trials. This
have been developed, providing a more the active ingredient, thus the pathogen dilemma has left the Chinese to find
potent effect in terms of disease control. will be negatively affected and the crop alternate MoAs to control canola
The application methods and protected. In contrast, pathogens that are stem rot, as the effectiveness of the
optimised timing of fungicides have not affected by the fungicide are known dimethachlon is threatened by the
evolved alongside these technologies, to be ‘insensitive’ or ‘naturally resistant’; resistance observed (Ma et al., 2009).
in combination with an integrated in such instances the product will not
pest management strategy, using provide sufficient protection to the host. Local considerations
complimentary non-chemical When an active ingredient or product Looking at these two examples, it is
interventions to minimise the effects with a similar mode of action (MoA) or critical that fungicide resistance to local
of crop diseases. While hundreds of mixture is used for prolonged periods, pathogens, local environments and
fungicidal compounds exist, few target the pathogens may no longer be local crop genotypes be considered.
crop-pathogen specific needs effectively effectively controlled by a once adequate In Brazil, fungicide resistance to
and reliably (Brent and Hollomon, 2007). product (Brent and Hollomon, 2007). procymidone was rare in contrast to
In this instance, the pathogen ‘acquires observations in China, where an active
Registered active ingredients resistance’ through the emergence, ingredient from the same fungicide
In South Africa, there are limited registered selection and adjustment of pathogen group indicated fungicide resistance
active ingredients to manage diseases genetics to the pressure imposed on and did not provide sufficient control.
caused by Sclerotinia sclerotiorum. them to survive (Brent and Hollomon, The Fungicide Resistance Action
Benomyl is registered as a sunflower 2007; van den Bosch et al., 2011). Committee (FRAC) was established in
seed treatment. The remaining active 1994 to address issues surrounding
ingredients listed are recommended Managing pathogen resistance resistance management and provide
for application at early bloom – ~1 Field resistance of plant disease fungal countermeasures to reduce or delay
to 20% flowering depending on the pathogens is known to develop against resistance. Fungicide resistance is a
crop. Procymidone is registered for fungicides, threatening the effectiveness complex and devastating occurrence –
application on dry bean, green bean, and future use of products (FRAC, 2019). likely inevitable in modern agricultural
soya bean and pea. The management S. sclerotiorum has varying sensitivity practices – and should be considered
of Sclerotinia stem rot of canola is to fungicide application in the field. by all crop management specialists
possible with either azoxystrobin or a In Brazil, procymidone is used when developing a disease control
combination of prothioconazole and extensively to control Sclerotinia rot of system (Brent and Hollomon, 2007).
tebuconazole, while leafy vegetables, the common bean. Brazilian isolates
such as lettuce, can be controlled of S. sclerotiorum indicated low to rare References available on request.
with a combination of cyprodinil levels of resistance; however, resistance For more information, contact
and fludioxonil (AVCASA, 2018). management practices were suggested Lisa Rothmann at info@sclerotinia.co.za,
Registered products are applied to reduce the likelihood of the spread visit www.sclerotinia.co.za or follow the
at different application frequencies of fungicide resistant genotypes of South African Sclerotinia Research Network
in fields due to the nature of disease the pathogen (Lehner et al., 2015). on Facebook, Instagram and Twitter.

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