Dr Belinda Janse van Rensburg, a researcher at the Agricultural Research Council’s (ARC) Grain Crops Institute (ARC-GCI), notes that while there are a host of diseases that affect groundnuts, the most important are early leaf spot and sclerotium stem rot.
“Both of these diseases are difficult to control and are particularly devastating when the weather is warm and the soil is moist,” she explains.
An additional infection to avoid is that of aflatoxins. While they don’t impact yield, the repercussions of such an infection are considerable. When crops that contain aflatoxins are consumed by humans or livestock, they can have substantial negative health effects.
Contaminated groundnuts consequently need to be removed, which reduces the size of the marketable crop. Farmers therefore need to implement strategies to prevent this fungus from taking hold in their crops.
The right start
Diseases in groundnuts can be classified as leaf, stem, and pod diseases and, in addition to these, particular viral diseases are also encountered. Management includes chemical and biological applications, cultural management, and breeding for resistant varieties.
A key factor preventing diseases in groundnuts is crop rotation, the best of which is where a grass fallow is followed by groundnuts.
According to research done by the ARC-GCI, fewer diseases are present in groundnuts following a grass crop. Groundnuts also generally produce a better crop on fields that have been fallowed.
Groundnuts should not follow cotton, soya bean, or tobacco due to the risk of diseases being carried over from one season to the next.
However, where farmers have no choice other than to plant groundnuts after one of these crops, the ARC-GCI recommends that deep ploughing be done to remove excess plant material from the surface, as it acts as a disease host.
While minimum tillage has become more prevalent among grain farmers, the lack of ploughing greatly inhibits groundnut yields, since the soil layer where the pods develop should be devoid of plant residues. Where plant residues are present, pod rot diseases increase, which influences yield and quality.
Sclerotium stem rot
A disease that has not traditionally plagued farmers in South Africa is Sclerotium rolfsii (Athelia rolfsii), or sclerotium stem rot. However, with the wetter weather during autumn, Janse van Rensburg notes that the ARC-GCI has been inundated with requests for advice on managing this disease.
Since it can survive in the soil for many years, care should be taken to properly manage the disease this season to prevent its proliferation in the next.
S. rolfsii normally infects the lower stem of the plant near the soil surface. In some plants, it may infect the roots, and on low-growing plants like groundnuts, leaves touching the soil may also be infected.
The disease occurs worldwide but more commonly in warmer (above 24°C), moist climates. Although it has been widely researched, control remains challenging.
The first symptoms of infection are small, water-soaked lesions on the lower stems. These lesions spread and finally girdle the stem of the host plant, which then results in wilting. The wilted leaves turn yellow and then brown, but generally remain hanging on the plant.
As the lower infected portions of the stem begin to decay, a white mat of mycelium develops around the lesion site. This mycelial mass may often spread out onto the nearby soil surface.
Sclerotium stem rot thrives in highly aerobic environments and therefore survives best on or near the soil surface. Wounds caused to plants by hoeing, mechanical machinery moving through the land, nematodes or insect damage increase the possibility of infection.
Cultivation during the season should therefore be avoided.
“Cultivation can promote disease by uncovering buried sclerotia, improving aeration, and by throwing infested soil onto plants. Furthermore, aeration of the soil allows deeper penetration of the fungus where roots, pegs and pods can be infected,” says Janse van Rensburg.
She laments that control of this pathogen is difficult as it produces sclerotia, which overwinter in the soil to emerge as inocula and cause disease the following season.
“Various chemical, biological and cultural control strategies have been suggested and implemented, some of which have reduced disease incidence in the field. No groundnut cultivars are completely resistant to infection, and where the disease is known to be a problem, fungicides may be used to control the disease.”
Soil tillage prior to planting groundnuts buries sclerotia and thus reduces inoculum levels.
Janse van Rensburg says that generally, a mouldboard plough will bury inocula deep enough.
“Sclerotial germination has been found to be lower at depths below 2,5cm than at the soil surface, and germination is nil at depths of 8cm or more.”
While crop rotation is an effective practice, farmers must ensure the crops are not hosts of the fungus as it has a very wide host range.
Maize, grain sorghum, small grains, Sudan grass, and cotton are possible options for suitable rotation crops.
A minimum two-year rotation is required where the disease pressure has been high.
Leaf spot
A notable disease of groundnuts is leaf spot – both early and late – although it is far easier to control with a proper pesticide programme. Both are expressed on the leaves and stems, resulting in poor crop stand and yields if left unmanaged.
Early leaf spot develops small necrotic flecks that usually have light to dark-brown centres, and a yellow halo. The spots may range from 1mm to 10mm in diameter.
Late leaf spot develops small necrotic flecks that enlarge and become light to dark brown. The yellow halo is either absent or less conspicuous than in early leaf spot. What makes late leaf spot challenging to manage is that its symptoms can be confused with harvest indicators.
Robert Tumusiime, a researcher at the National Groundnut Improvement Project in Uganda, recommends cultural and chemical options for control.
“Crop rotation has been shown to provide partial control of leaf spots. When groundnuts are followed by either maize or pasture, the disease development is slow and less severe,” he says.
Early sowing has been shown to reduce the severity of leaf spot diseases. Farmers should therefore adjust the date of sowing to avoid conditions favourable for rapid disease development.
Burying all groundnut crop residues by deep ploughing will also reduce the presence of initial inocula.
Farmers have multiple options for fungicides to control leaf spot. Tumusiime recommends three sprays of 0,2% chlorothalonil at intervals of 10 to 15 days starting 40 days after germination, up to 90 days.
“Applications of fungicides such as benomyl, chlorothalonil, copper hydroxide, mancozeb or sulphur fungicides may control early and late leaf spot. However, carbendazim (0,05%) controls both leaf spots very effectively,” says Tumusiime.
Aflatoxin
Aflatoxin contamination is caused by the fungus Aspergillus spp. Unfortunately, no groundnut variety is completely resistant or immune to it, although there are some that offer moderate resistance. Contamination can occur in the field before harvest and after harvest during curing, storage, and transportation.
Aspergillus spp. infection and aflatoxin contamination are more evident where there are high temperatures, high moisture, moisture stress, poor harvest- and post-harvest management, and inadequate storage structures.
Janse van Rensburg recommends agronomic practices such as crop rotation, use of resistant varieties, insect and weed control, timely planting and harvesting, adequate fertilisation, and late-season irrigation to reduce preharvest aflatoxin production.
A more promising, preventive approach is however emerging in the form of a biocontrol that uses atoxigenic strains of A. flavus and/or A. parasiticus in groundnut fields to outcompete the naturally occurring toxin-forming strains. Janse van Rensburg notes that this biocontrol, Aflasafe, is the most successful method used to date for the control of aflatoxins in Africa.
“Additionally, atoxigenic Aspergillus strains can also be applied in the field to competitively displace and reduce the toxigenic Aspergillus population in the soil. Moreover, these atoxigenic strains additionally provide a carry-over effect that can protect groundnuts from postharvest aflatoxin contamination.”
Results to date indicate that all Aflasafe products, registered and under experimental use, reduce aflatoxin concentrations in treated crops by more than 80% compared to those not treated under both field and storage conditions. Janse van Rensburg adds that even though Aflasafe is a viable solution to the aflatoxin challenge, it works best when applied in combination with other good practices such as proper drying and storage of the groundnuts.
However, Aflasafe technology doesn’t exist specifically for South African farmers using non-toxic South African strains. The ARC-GCI, in collaboration with the Department of Agriculture, is therefore attempting to develop an Aflasafe product specifically for South African farmers.
“More than 2 000 fungi isolates from different groundnut-growing areas across the country need to be screened. We need farmers to help us by submitting pod samples for fungal isolation. Hopefully, it will be possible to identify non-aflatoxigenic strains from these isolates to be included in a product tailor-made for South Africa,” says Janse van Rensburg.
Farmers who can assist with sample collections may email Belinda Janse van Rensburg at [email protected].
