HANLIE DU PLESSIS for SANSOR
Sclerotinia, a sporadic fungal disease affecting a wide range of host plants, has had a substantial impact on soybean production during the previous season. This fungus has the potential to reduce soybean yields with up to 50%, should conducive environmental conditions for the infection and rapid development of the disease prevail during the growing period.
Unfortunately, by the time the first Sclerotinia symptoms are observed, very little can be done to control this fungus. Therefore, preventative management practices should be implemented and applied to minimise the level of infection and further spread of the disease.
One of the early symptoms of Sclerotinia disease is the sudden wilting of soybean plants. Leaves will turn greyish-green before turning brown, curling up and eventually dying off. The fact that the leaves do not drop immediately, makes it easier to distinguish infected plants from healthy ones.
Symptoms will normally be visible within two to three weeks after flowering has commenced in the crop, given that the right conditions persisted, and infection has occurred.
Sclerotinia can easily be distinguished from most other soybean diseases by the presence of a white, cottony, mouldy growth and hard, black sclerotia on the inside and outside of infected stems and pods. The black sclerotia is the survival structure of Sclerotinia and can easily be spread during harvesting.
The fact that sclerotia can survive in the soil for a period of between four years to seven years, while awaiting favourable conditions, makes it even more difficult to contain this disease. The most favourable conditions for the fungi to develop occur during the flowering stages, with high rainfall and cool temperatures of below 28˚C.
The development of the disease occurs primarily after the closing of the leaf canopy, which promotes cool temperatures and creates a humid micro climate around the stems of the plants. High soil moisture as a result of rain or irrigation contributes significantly in creating the favourable micro climate underneath the closed canopy.
Sclerotinia spores and sclerotia are widely found across South Africa. However, some management practices conducted by producers may contribute towards the build-up of these bodies in our soils.
When grain, which was harvested the previous season, is being used as seed it may not have been subjected to testing for quality properties. Should the commercial production have been infected with Sclerotinia, the sclerotia which was harvested together with the grain then gets planted together with the farm-saved soybean ‘seeds’.
There are different opinions on the reasons for the sudden increase in Sclerotinia occurrence. Mr Whitey van Pletsen (production manager of Agricol and chairman of SANSOR’s Seed Certification Standing Committee), holds the opinion that the only effective way to manage Sclerotinia is by following an integrated process of best farming practices.
Insist on certified seed
Start by planting healthy, good quality certified seed. The fields on which certified seeds are produced by seed companies, are subject to regular inspections throughout the process of planting, production, harvesting, cleaning and packaging.
Seed samples of registered seed lots are taken by authorised seed samplers for various tests to be conducted. Field inspections, sampling and quality testing must be done according to internationally accepted, validated methods and procedures. Seed testing is important to assess the quality of seed being marketed to producers.
Mr Kobus van Huyssteen (technical officer at SANSOR) explains the thorough and lengthy process of seed certification, ‘Certified seed must meet more and stricter requirements than other seed. The process, with more than 90 control points, exercises control from breeder seed, through pre-basic, basic and finally to certified seed multiplications and aims specifically to guarantee varietal purity and varietal identity, as well as seed of good physical quality.’
SANSOR will only certify seed lots that have been produced on fields registered with them and produced according to the specifications and requirements of the SA Seed Certification Scheme. During the registration process the origin of the seed is verified to ensure that it is acceptable for certification.
Field inspections, during which several aspects such as varietal purity and isolation distances are controlled, are conducted by trained and authorised seed inspectors. After harvesting, processing and packaging, the seed is sampled and tested by registered seed testing laboratories to assess the germination potential and physical purity of the seed lots.
The percentage of sclerotia in seed samples is determined by using the methodology prescribed by the International Seed Testing Association (ISTA) Rules for Seed Testing, at the same time as when general quality testing is conducted by a registered seed testing laboratory. The presence and level of occurrence of sclerotia (or seed that has been transformed into visible fungal sclerotia) in a seed sample is determined by the seed analyst, in the process of conducting a standard ISTA purity analysis. The virulence of the sclerotia is not determined during these examinations.
Certified seed lots must comply with the minimum physical requirements as stipulated in the South African Seed Certification Scheme. The scheme states that a sample of 1 000 g shall not contain more than 0,2% sclerotia of Sclerotinia sclerotiorum, calculated on a total weight basis. The sample must be taken by an authorised sampler in the methodology prescribed by ISTA to ensure that it is representative of the seed lot.
The results obtained are reported on a Report of Analysis, issued by the registered seed testing laboratory and submitted to SANSOR for assessment and approval before a final seed lot certificate will be issued.
Post control grow-outs are planted by the Department of Agriculture, Forestry and Fisheries (DAFF) to verify the varietal purity and identity of the production unit.
Mr BP Greyling, a mega producer and Agricultural Writers’ SA Farmer of the Year in 2011, farms on Langfontein near Wakkerstroom in Mpumalanga. He shared his experience on the use of certified seeds, ‘I have planted soybeans for the past 32 years. I won’t say that I never had Sclerotinia on my farm, but I can say that in 32 years, I never had Sclerotinia on any field where I planted certified seeds’.
He is convinced that buying certified seeds is the first and most important step towards keeping soybean fields Sclerotinia-free.
Good management practices
Good management practices used by seed production companies against Sclerotinia infection:
A number of varieties has proven to be either more tolerant or more susceptible to Sclerotinia infection than others. This information, obtained through internal product research, can usually be obtained from the seed companies marketing those varieties.
There is currently no soybean variety known to be completely resistant to Sclerotinia. The use of more tolerant varieties is an effective tool in managing the disease.
Avoid planting highly susceptible varieties in fields with a history of Sclerotinia infection.
Plant population and general planting direction
Unfortunately, producers follow production practices that can often create the ideal micro climate and environment for diseases to develop. Some of the following practices are associated with high infection rates: Narrow row spacing, high seeding rates, late planting dates, and the over application of nitrogen.
Cultivation practices used should discourage the formation of a cool and humid micro climate wherein Sclerotinia can thrive. Wider rows, and planting from north to south, will allow sunlight and the movement of air into the rows, especially after irrigation or spells of rain.
Soybean plants are very adaptable and known to compensate in cases where lower seeding rates are observed.
Sclerotinia has a wide host range including soybean, sunflower, dry bean, potato, pea, cucumber and some common weeds, to name a few. Crop rotation towards non-susceptible crops such as maize and sorghum can help to reduce the level of sclerotia occurrence in the soil. Rotation crops need to be planted for at least three consecutive planting seasons before soybeans can be reconsidered on the same field.
Several broad-leaved weeds are known to be Sclerotinia hosts and aids in the spread of the disease. A well-planned spray programme should be followed as recommended by chemical sales agents.
Fertility and plant nutrition
High soil fertility, especially the use of nitrogen-rich manures and fertilisers, favours Sclerotinia development by promoting lush plant growth and early canopy closure. Having soil fertility tests conducted on a regular basis will help avoid over-fertilising fields that are prone to Sclerotinia infection.
Excessive irrigation, above what is needed to maintain yield potential, should be avoided during flowering to minimise moisture at the soil surface and below the crop canopy. Low moisture levels within the soybean canopy are critical for reducing the potential for infection.
Occasional, heavy watering is better than frequent, light watering. Avoiding excessive irrigation is especially important during the critical periods of infection from early flowering to early pod development stages.
Chemical applications can be a component of an integrated management system for Sclerotinia. Some foliar-applied fungicides and herbicides have efficacy against Sclerotinia, although none of them offers complete control.
As previously mentioned, sclerotia often get distributed by combine harvesters. It is therefore imperative to harvest the fields where the fungus was observed, last.
Further research is needed to effectively manage this dreaded disease, with a special focus on the breeding of new, more resistant cultivars, biological control agents and integrated disease management systems.
Publication: December 2018
Section: On farm level