By Dr. Rikus Kloppers

Goss’s wilt and leaf blight is caused by Clavibacter nebraskensis, a bacterium that derives its name from the fact that the disease was first noted in the US state of Nebraska in 1969. The bacterium is therefore neither new nor unknown and has also been around in South Africa for a long time, albeit in other guises, such as tomato wilt that is caused by Clavibacter michiganensis subspecies michiganensis.

Goss’s wilt in SA

In South Africa, Goss’s wilt was first noted and reported early in the 2024 season. Most cases were found in Northwest, with some unconfirmed reports in Gauteng and on the Eastern Highveld.

However, it is possible that the disease has been in the country since the early 2000s but that it remained undetected due to its symptoms being similar to those caused by other disease or stress conditions and because it was not widespread.

Despite the first positive confirmations taking place in February 2024, the Department of Agriculture only officially declared the disease’s presence in SA by the end of the year. Any potential new disease must be carefully investigated and identified without doubt in view of the phytosanitary implications for international trade in seed and commodities. These concerns became reality when Botswana and Namibia immediately suspended all imports of maize, sorghum and wheat grain and seed as soon as the International Plant Protection Convention (IPPC), to which SA is a signatory, issued an official statement on 14 January 2025 that Goss’s wilt had been confirmed in SA.

To the local grain industry’s great relief, the Department of Agriculture, and role players such as GSA, managed to convince our neighbours that grain constitutes a low disease transmission risk and the ban on grain imports was lifted in its entirety. Seed exports, however, remain subject to certain conditions.

The Department of Agriculture still considers Clavibacter nebraskensis to be a regulated organism under its control and surveys are currently underway to determine the distribution and extent of Goss’s wilt. In support of this effort, growers are advised to report disease occurrences to their representatives. There is no risk whatsoever that farms will be quarantined and prevented from delivering their grain.

What growers need to know
The bacterium that causes Goss’s wilt is carried by seed from one continent to another but, within an area, distribution only happens when infected crops are harvested, and bacteria become airborne and are blown onto adjacent fields. The inoculum settles in plant debris that becomes the source of infection in the following season, rapidly infecting susceptible hybrids under ideal growth conditions. Insects are not known vectors for this disease.

Grain harvested from fields in which Goss’s wilt and leaf blight occurred is safe for human and animal consumption and can be delivered without any grading complications.

This does not mean, however, that the disease is harmless. In heavily infected fields, yield losses can reach up to 50% in highly susceptible germplasm when conditions are favourable, and hail or wind damage occurs. The latter provides the plant lesions in which bacterial infections develop and flourish. Fields with previous Goss’s wilt infections may not see significant disease development without weather-related injuries. For this reason, and due to disease resistance in maize hybrids, Goss’s wilt is regarded as only the fifth most important maize disease in the US.

Yield losses are primarily due to the disease’s leaf blight phase, which reduces the photosynthetic area and causes premature plant death. The disease grows cumulatively severe in subsequent years as the inoculum load increases in crop residue and alternative hosts, such as grasses. Inoculum from the crop residue infects the new crop and spreads through the canopy by means of rain splash, irrigation and wind. Early infections lead to significant yield losses, while late infections often have minimal impact.

Symptoms of Goss’s wilt and leaf blight

Oblong or elongated dead, white lesions/strips.
Water-soaked, greyish green tissue where bacteria actively grow.
Irregular dark green or black spots or freckles often near lesion edges.
Sticky, shiny exudate (ooze) in the streaks, which dries to form a glistening residue, or varnish, within the lesion.
Lesions coalesce and lead to complete foliar demise.
When infected systemically in the seedling stage, plants have discoloured vascular tissue with a slimy bacterial exudate in the stalk.
Plants are stunted, wilt and die as if drought stressed.

The symptoms of certain other maize foliar diseases, notably northern leaf blight, or environmental stressors, specifically sunburn, heat and drought, can easily be confused with Goss’s wilt. It is therefore important that growers familiarise themselves with these diseases and stressors and/or to consult an expert plant pathologist to confirm the cause of symptoms. Laboratory tests are highly accurate and worth doing if the disease diagnosis is in doubt.

Prevention is the only cure
Infection cannot be undone as no chemical products capable of controlling Goss’s wilt exist. Prevention is therefore vital. Experts recommend these measures:

Plant high quality, uncontaminated seed from resistant hybrids as the primary management method.
Reduce maize residue through crop rotation with soybean and sunflower and till fields to bury infected debris.
Manage grassy weeds that can act as alternative hosts for the bacteria.
Clean equipment to avoid spreading the pathogen to uninfected fields.

Hybrids that are resistant to Goss’s wilt are well established in the US, and resistance currently evident in SA hybrids is the indirect result of the exchange of high-potential germplasm between the two countries.

Corteva’s breeders, experienced in developing resistant hybrids in the US, are working with South African breeders to establish resistance in locally adapted breeding material. Trials done during the current season in areas that suffered severe infection in the previous season, are currently being evaluated for resistance. Pannar will share the outcomes with growers in the near future.

It is important that growers do not focus all their attention on this one disease. Northern leaf blight, grey leaf spot, rust and ear and stalk diseases remain realities, requiring growers to follow a bundled approach to responsibly spread their risk.

BUSINESS REPORT

#maize

Goss’s wilt and leaf blight – a new disease in South African maize production