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At Pannar, we understand that every successful harvest starts with knowledge and preparation. Goss’s Bacterial Wilt and Leaf Blight can be a formidable challenge, but with the right insights and strategies, we can help you protect your fields and secure your yield. Together, let’s take a closer look at this disease and explore the steps you can take to keep your crops thriving.
Goss’s Wilt is caused by the bacterium Clavibacter nebraskensis (previously known as Clavibacter michiganensis subsp. nebraskensis). This gram-positive, rod-shaped bacterium appears in pairs or short chains under the microscope. In culture, its colonies are yellow to orange-pigmented, and the bacteria appear blue/purple under the microscope when gram-stained. The disease is also known as Leaf Freckles and Wilt or Nebraska Leaf Freckles and Wilt.
Goss’s Wilt was first identified in Nebraska, United States, in 1969 and has since spread eastward across the corn belt, including the upper Midwest and western Canada. It was reported in South Africa during the 2024 season, mainly in the North West province, but there were also reports from the Eastern Highveld. Due to its symptoms being similar to other diseases or stress conditions, it might have gone undetected for several seasons, especially during drought conditions.
In heavily infected fields, yield losses can reach up to 50% in highly susceptible germplasm that experiences hail or wind damage. In the USA, Goss’s Wilt is regularly among the top three of the 10 most destructive maize diseases. The disease can reduce plant stand and vigour, as well as stalk and grain quality. During the systemic infection phase, it reduces plant stands and weakens surviving plants. Yield losses are primarily due to the leaf blight phase, which reduces the photosynthetic area and causes premature plant death. The disease can be more severe in the year following an infection due to increased inoculum in crop residue. Early infections lead to significant yield losses, while late infections often have minimal impact. Secondary effects include poor standability and lodging, leading to harvest losses or poor quality grain.
Due to its restricted occurrence in the world and its seed-borne aspect, it also has a phytosanitary impact on seed and grain trade to areas where the disease is not present. Clavibacter nebraskensis has previously only been found in North America and should be considered a significant quarantine risk. However, this risk is only moderate because extensive resistance exists in modern maize inbreds and hybrids. The inefficiency of seed transmission is one reason for the limited spread of the pathogen within the USA since it was first reported in Nebraska in 1969.
A symptom is the response of the plant to a disease infection (e.g. necrotic lesion), while signs refer to the actual presence of the disease-causing organism (in this case, the bacterial cells oozing out of the stomata). Typical symptoms include leaf blight and systemic wilt phases.
The leaf blight symptoms are visible in the early stages as oblong or elongated lesions of water-soaked, greyish-green tissue that progress to long dead streaks. These wavy lesions have water-soaked margins with dark green or black freckles (water-soaked, irregular spots, often near lesion edges) within the lesions. An additional typical sign is the formation of a sticky, shiny exudate (ooze) in the streaks, which dries to form a glistening residue, or varnish, within the lesion. Streaks also extend along the leaf veins, which suggests a bacterial infection, and these could easily cross leaf veins, resulting in unusually wide lesions for a bacterial disease. The lesions coalesce when infection is severe and may lead to rapid foliar demise and premature death of the plants. Major loss of leaf area may lead to the plants robbing sugars from the stalk, which can lead to loss of stalk integrity (lodging), stalk rots, and ear moulds. This dramatic loss of photosynthetic area will often result in yield loss in the form of nosed-back ears, rubbery ears/cobs, and loss of test weight. Plants may also be infected systemically by Goss’s Wilt, especially in the seedling stage. These plants have discoloured vascular tissue with a slimy bacterial exudate in the stalk. Plants are commonly stunted, wilt, and die as if drought-stressed. Seedlings and early V-stage plants may acquire the bacteria systemically by hail damage or sandblasting.
There are several maize leaf diseases and environmental stress conditions that produce similar symptoms that can easily be confused with Goss’s Wilt. It is therefore important to familiarise oneself with these and/or to consult an expert plant pathologist to confirm by means of visual expertise combined with laboratory confirmation. The main diseases that are often confused with Goss’s Wilt are:
Goss’s Wilt overwinters in crop residue and grass species. The disease thrives after hailstorms or sandblasting, as bacterial invasion requires plant injury. Inoculum from the crop residue causes infection and spreads through the canopy by rain splash, irrigation, and wind. Maize plants are at the greatest risk of yield loss at early grain fill under severe leaf blight pressure. Wet weather and high humidity encourage its development.
There are two disease phases, the less common systemic wilt and the more prevalent foliar blight.
Fields with previous Goss’s Wilt infections may not see significant disease development without weather-related injuries.
Preventing infection is crucial, as no rescue measures exist. Key management strategies include:
Resistance: Use resistant hybrids. This is the primary management method.
Residue Management: Reduce corn residue through crop rotation and tillage.
Weed Control: Manage grassy weeds that host the bacteria.
Prevention/Avoidance: Clean equipment and avoid spreading the pathogen to uninfested fields. Plant uncontaminated seed in areas without a history of the disease.
No chemical products are available to control Goss’s Wilt. Foliar fungicides for fungal pathogens are ineffective since Goss’s Wilt is caused by a bacterium. There are no chemical products that can be applied to control the bacterium itself. Copper compounds used in horticulture are not economically feasible for maize due to the need for frequent applications. There are also no seed treatments to manage the risk of spreading disease through seed.
Information on resistance in South African commercial maize hybrids is limited. Although more damage has been reported on white hybrids, some yellow hybrids have also been affected. More white hybrids are grown in the west where the outbreak was first observed. Initial observations indicate hybrid differences, but true resistance requires consistent testing. Unless hybrids are compared under the same disease pressure and environmental conditions, we cannot confirm true resistance. It could easily happen that a susceptible hybrid was planted in an area where there was lower inoculum pressure and/or no hail or wind damage that could facilitate infection. That would be considered disease escape and not true resistance. Corteva’s breeders, experienced in developing resistant hybrids in the USA, are working with South African breeders to improve local resistance. This collaboration aims to enhance the Goss’s Wilt resistance in South African maize hybrids.By having a better understanding of the disease and following these guidelines, farmers can effectively manage and mitigate the impact of Goss’s Bacterial Wilt and Leaf Blight on their maize crops.
Goss’s Baterial Wilt and Leaf Blight Fact Sheet, Version 1. Copyright © 2024 Pannar Seed (Pty) Ltd. Compiled by Dr Rikus Kloppers and Dr Scott Heuchlin. Comments/feedback can be sent to infoserve@pannar.co.za. Disclaimer: This document serves as a guideline and is given in good faith.
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