By Charl van Rooyen
Early observations indicate that certain South African maize hybrids may already have some resistance to Goss’s bacterial wilt and leaf blight – commonly known as Goss’s wilt. The good news for maize farmers is that trials are currently under way in affected areas to evaluate this resistance, with findings expected before the next planting season. This information, combined with proven management methods, could help farmers better combat the disease.
Dr Rikus Kloppers, an independent crop disease specialist, told African Farming that seed companies are conducting cultivar trials in regions where the disease has occurred. Disease readings are also being taken during these trials, and once the results have been processed, recommendations will be made to maize farmers.
The United States has been breeding maize hybrids that are resistant to Goss’s wilt – caused by the Clavibacter nebraskensis bacterium – for about 30 years. As a result, there is a strong likelihood that some imported hybrids in South Africa already carry resistance. Since South Africa collaborates with multinational seed companies, germplasm (maize genetic material used for breeding and developing new hybrids), some of which is already available locally, can be exchanged. Consequently, certain hybrids in the country are already showing resistance to Goss’s wilt. South African breeders can now build on this foundation to develop locally adapted cultivars with built-in resistance.
Dr Kloppers still recommends that farmers spread their risks by planting a variety of hybrids, as Goss’s wilt is only one of a range of maize diseases they have to combat. Other major diseases include grey leaf spot, northern corn leaf blight, and Diplodia ear and stalk rot. Farmers need to make informed choice about cultivars based on the risk of specific diseases in their region, he says.
Good news for farmers
The recommendation of resistant hybrids will come as a relief to concerned farmers following the alarming news that Goss’s wilt has spread from the United States to South Africa.
Farmers in several provinces have already suffered damage, including Cobus van Coller, a grain farmer from Viljoenskroon, Free State. He told African Farming that he noticed a problem in a 200-hectare maize field on one of his farms last season but was unsure of the cause. He told his crop advisers that it wasn’t stem rot or drought symptoms – it had to be something else.
“At the time, we were still unfamiliar with the new wilt disease, later identified as Goss’s wilt. It hit me hard. The yield from that field was 1,5 tonnes per hectare lower than the rest of my maize.”
Van Coller rotates maize with soya beans, and the disease has not reoccurred in that field. It now appears, however, that one of his neighbours’ maize fields is affected by wilt disease, prompting him to inform a seed specialist.
Dr Kloppers says Goss’s wilt, which can be mistaken for sunscald, leads to a reduced yield, similar to what a farmer might experience with northern corn leaf blight. In the worst-case scenario, crop losses of up to 50% could occur if conditions are particularly favourable for the disease, although this is not typical.
Keeping farmers up to date
Dr Kloppers spoke about Goss’s wilt at a series of Pannar Seed farmers’ days. As a seed-borne disease, it likely spread to South Africa through contaminated seed – possibly from a commercial seed lot or even “a few seeds in someone’s pocket”. Blaming anyone is unproductive, he says, as the exact source of the contamination remains unknown.
At a farmers’ day in Val, Mpumalanga, Dr Kloppers said that Goss’s wilt was causing significant unease and uncertainty among farmers due to the many “theories and stories” circulating about it.
The disease first emerged in Nebraska in 1969 and gradually spread throughout the Corn Belt in the Midwestern United States. Over time, farmers in affected areas learnt to manage the disease through resistance and effective farming practices, which has significantly reduced its impact.
Key steps to follow
Dr Kloppers recommends the following steps:
1. Identify the disease.
2. Harvest the disease-affected field last, and clean the combine thoroughly before moving it to another part of your farm. The bacteria can be spread by dust.
3. If a field was heavily infested, use deep tillage to incorporate the stover – leftover stalks, leaves, husks and cobs – into the soil. The Clavibacter nebraskensis bacterium does not survive as long as some other pathogens (such as Sclerotinia, which can persist in soil for up to 20 years). The remains of the bacteria break down in the soil, eventually disappearing. No-till farmers might not like it, Dr Kloppers says, but diseases sometimes require drastic measures.
4. Plant soya beans, sunflowers or peanuts in that field the next season. These crops won’t be affected by or serve as a host for the bacterium. Goss’s wilt primarily affects maize, and sorghum, sugarcane and certain grasses could be secondary hosts.
5. In the following season, plant a maize hybrid that is resistant to Goss’s wilt.
Not a man-made disease
Dr Kloppers says some people claim that Goss’s wilt was “created by chemical companies to make money”. But aside from the fact that Clavibacter is an old, well-known maize disease, no chemical agent has been registered to control it.
The only substance that provides some control is copper, but it is not really effective since too many applications are required – the volume of maize grown makes it economically unfeasible.
When asked why the disease hasn’t been eradicated, Dr Kloppers explains that it’s an incredibly small microorganism that couldn’t be detected at an airport, for example, like a few seeds or fruits. “In today’s global environment, it wasn’t a question of if Goss would occur here in South Africa, but when.”
He suspects the disease arrived here as early as 2020. It went unnoticed at first because it’s easily confused with other problems.
Dr Kloppers received a report from a farmer who suspected he had Goss’s wilt on his farm in February 2022. At the time, he didn’t think it was possible because the disease had never been identified in South Africa. In February 2024, the same farmer reported that he was certain Goss had infected one of his maize fields, and this was later verified.
Questions & answers
Dr Kloppers answered farmers’ questions.
What happens if Goss is found in my maize?
The Department of Agriculture, Land Reform and Rural Development has confirmed that farms will not be quarantined. You don’t have to worry about your farm being sealed off, your equipment being locked away, or being prevented from selling your grain. There is no evidence that Goss’s wilt can be controlled through quarantine measures. If you suspect the disease is present on your farm, contact a seed specialist or agricultural adviser for guidance.
Will my maize field be destroyed?
No, it won’t.
Can cattle spread the disease?
There is a possibility. As far as cattle manure is concerned, if it’s quickly incorporated into the soil and decomposes rapidly, the bacteria are unlikely to survive for long.
Will burning maize stover help?
Burning may serve as an emergency measure in cases of severe infestation, but it is not considered good agricultural practice.
Does plant density affect the spread of the disease?
No, it does not.
Could fertiliser burn during top dressing create an opportunity for infection?
Yes, the damage can create an entry point for the bacteria.
Crop-protection reps are already recommending a chemical product against Goss …
There have been such rumours, but no product has been registered yet. There will be a solution one day, but it will be a biological product rather than a chemical one. People are probably trying anything now to see whether it will suppress Goss.
