Fungicides are for Fungi
Management of foliar fungal disease is achieved in many of our field crops by applying fungicides. Over the past several years, there have been examples of misidentification of some bacterial diseases that are easily confused for fungal diseases in field crops. This article reviews some of the common mistakes made in the major field crops and reviews research on the impact of fungicide use after hail events in corn and soybean, including:
- comparison of identification of fungal diseases and common bacterial diseases that can be confused in Nebraska field crops;
- effects of fungicides after hail injury: research data summary;
- factors affecting overall fungicide activity on late season crop health; and
- fungicide effects on yield in the absence of disease versus in the presence of fungal diseases.
Bacterial Leaf Streak versus Gray Leaf Spot
Bacterial leaf streak can appear very similar to the common fungal disease, gray leaf spot. Misdiagnoses have led to fungicide treatment of bacterial leaf streak that won’t control the pathogen. Look closely at lesions for wavy, jagged edges indicative of bacterial leaf streak versus the smooth rectangular lesions of gray leaf spot. Bacterial leaf streak also may appear as bright yellow when backlit and develop during the early growing season, in contrast to gray leaf spot.
Goss’s Bacterial Wilt and Blight versus Northern Corn Leaf Blight
Goss’s bacterial wilt and blight was more common in 2017 than in recent years. Lesions often have small, dark “freckles” and a water-soaked appearance on the edges. In contrast, fungal diseases, such as northern corn leaf blight and Diplodia (Stenocarpella) leaf streak can also produce large lesions, but lack the water-soaked appearance common in bacterial diseases. Northern corn leaf blight commonly has lesions with rounded ends versus Diplodia leaf streak that often causes lesions with pointed ends.
Rust versus Sunburn
Rust is an important disease that affects dry beans in Nebraska, Colorado, and adjacent regions in the Central High Plains. The disease is caused by the obligate fungus Uromyces appendiculatus and has caused sporadic epidemics in this region for more than 50 years. Yield losses from the disease have been documented to exceed 50 percent, and a timely fungicide application will generally be effective in limiting yield reductions.
However, an abiotic condition has been noted in recent years that could easily be confused with rust. Leaves, stems, and pods affected by sunburn can look very much like rust from a distance, thus correct identification is critical to make a proper management decision. A misapplied fungicide will not affect the sunburn condition and will be economically damaging for the unneeded chemical and application costs. Rust will leave a powdery substance on fingers after touching symptomatic tissues; sunburn will not.
Cercospora leaf spot versus bacterial leaf spot
Cercospora leaf spot (CLS) is the most serious and destructive foliar disease of sugar beets in the Central High Plains of western Nebraska, northeastern Colorado, and southeastern Wyoming. This disease is caused by the airborne fungus Cercospora beticola. Individual lesions are approximately 1/8 inch in diameter with ash-colored centers and purple to brown borders, and are circular to oval shaped. Cercospora leaf spot is distinguished from other leaf diseases (Alternaria, Phoma and bacterial leaf spots) by its smaller size and shape and the presence of black spore-bearing structures, called pseudostromata, that form in the center of the lesions. These structures are easily seen as black dots with the aid of a hand lens (10 X magnification).
Correctly identifying CLS is also critical for making the most economical management decisions. It is important to distinguish it from bacterial leaf spot because use of a fungicide would not reduce disease caused by bacterial leaf spot. Furthermore, CLS is the only foliar disease that is potentially damaging enough to need to treat. Other fungal leaf spots diseases like Phoma and Alternaria do not generally cause enough damage to require fungicide treatments. Confusing CLS with other diseases is problematic in that it could lead to an unnecessary fungicide application or no fungicide application in the presence of CLS; producers would potentially lose either way.
Bacterial Streak versus Septoria Tritici Blotch
Bacterial streak (black chaff when it occurs on heads), caused by Xanthomonas campestris pv. undulosa, can be confused with Septoria tritici blotch caused by Zymoseptoria tritici. Lesions caused by both pathogens can enlarge into large brown necrotic areas or blotches on leaves that can be difficult to distinguish. Bacterial streak lesions are most conspicuous after heading, when many lesions can appear suddenly on the upper leaves without evidence of progression from lower leaves. They are irregularly shaped, elongate, and may extend the length of the leaf blade. When conditions are moist, bacteria ooze from the lesions and later the exudate dries to form thin flakes that are almost transparent and can be seen when the leaf is viewed at different angles.
In contrast to bacterial streak, Septoria tritici blotch lesions appear early in the spring on the lower leaves of seedlings that emerged the previous fall. On these seedlings, the lesions are oval with a tan center surrounded by a yellow halo. Asexual fruiting structures known as pycnidia usually will appear in these lesions as tiny black specks. Under wet conditions during stem elongation and heading, lesions appear progressively from the lower to the upper leaves. On the upper leaves, the sides of the lesions tend to be straight without a distinct yellow halo. The lesions may coalesce, resulting in large necrotic areas that in the absence of pycnidia can be indistinguishable from those caused by bacterial streak.