Understanding Bacterial Leaf Disease in Wheat

By David Boehm, AgriPro Wheat, David.boehm@agripro.com

What are Bacterial Leaf Diseases?

Bacterial diseases in spring wheat have become an increased topic of discussion for wheat producers and researchers in recent years. The question posed by some is whether yield or grain quality is affected and at what level. Bacterial leaf diseases are caused by two different pathogens, Xanthomonas translucens pv. translucens and Pseudomonas syringae pv. syringae. Both bacterial pathogens exist in other strains, and can survive in warm or freezing environments (1). Bacterial Leaf Streak (Xanthomonas) and Bacterial Blight (Pseudomonas) are the two diseases most referred to. Bacteria are ubiquitous in the soil and in host crop residue of the region. They are transferred to growing wheat, along with barley and rye, through openings in the plant surface, such as stomata or wounds caused by wind, mechanical or insect damage. Blowing wind or rain splash brings bacteria in the soil to the leaf surface (4).

Black chaff, also caused by Xanthomonas translucens pv. translucens, is defined by blotches or streaks on the peduncle or glumes which turn a dark brown to purple color (1).

Temperate and moist conditions favor infection. Moisture is needed to allow for bacterial infection and spread. Bacterial symptoms start with small, narrow chlorotic lesion on the leaf which turns yellow to dark brown while growing in size. When moist, lesions have a water-soaked appearance and sticky surface from the bacterial cells exudate. As lesions dry, a white or light colored crusty or scaly film remains from the dried bacterial cells. Lesions may have a translucent appearance (1,3,5). Bacterial diseases are commonly found in “hot spots”, where bacteria are transferred to neighboring plants by mechanical operations, insects or wind damage.

Does Bacterial Infection Cause Yield Damage?

Although there is limited research data regarding the relationship between bacterial infection and yield response, a growing belief among growers and researchers would suggest that a negative correlation exists in today’s environment. L.R. Waldron, in an NDSU study in 1929, did not find an adverse yield affect, but did report that black chaff caused a 10.5% reduction in grain test weight. He did report that it can reduce the number of grains per plant, wherein the disease has the potential to be more severe if plants are infected earlier in the season (2).

A 1987 study in Minnesota by Shane et al., reported a 8-13% reduction in test weight with a 50% flag leaf infection. A 100% flag leaf infection resulted in 13-34% test weight loss. No yield losses were reported in the 3-year study. (3).

In the summers of 2008 and 2009, yield losses attributed to bacterial infection by agronomists and producers were more common in fields considered to be significantly late planted. It would be reasonable to assume that these delayed, or younger, plants would have been infected at growth stages earlier as opposed to normal planting times. This would allow for greater infection in the plant and field, allowing for the possible yield losses proposed by Waldron. Growers have also told me that infection was greater after early wind events in less wind-protected fields and after heavy rain events. In these cases, growers have suggested yields losses caused by bacterial infection.

Are There Any Control Options for Bacterial Diseases?

There is no satisfactory control of bacterial disease or the prevention of infection. Crop rotation and tillage of infected stubble is of little value. Fungicidal seed treatment and foliar sprays have no effect on bacteria (4).

There are some varietal differences in tolerance. Plant breeders will screen for severity in their breeding programs, but can only do so when infection is present. In my opinion, and with the limited knowledge of bacterial effects, timing, or control, it would appear that the environment still controls if and how severe infection might be. Therefore, I wouldn’t propose varietal selection as a major management decision at this time.

An increased research focus is being proposed in regional universities. These include varietal scoring, bacterial strain DNA screening, and determining standardized methods of infection and screening. Potential losses in grain yield, and subsequently the economic benefit of controlling BLS would have to be established before increased research efforts are of value.

References Used:

1. The Bacterial Diseases of Wheat: Concepts and Methods of Diseases Management. CIMMYT: International Maize and Wheat Improvement Center. January 2000. www.cimmyt.org.

2. Waldron, L.R. 1929. The relationship of black chaff disease of wheat to certain physical and pathological characters. Science 70:268.

3. Shane, W.W., Baumer, J.S., and Teng, P.S. 1987. Crop losses caused by Xanthomonas streak on spring wheat and barley. Plant Disease 71:927-930.

4. McMullen, M.P., and Lamey, H.A. 1997. Symptoms and controls of crop diseases. NDSU Extension Bulletin PP-533.