Recognizing Leaf Spots and Reacting Correctly, Part 2: Biotic Leaf Spots

Radiation, fungal pathogens, or nutrient deficiency - all can lead to leaf spots and thus restrict photosynthetic performance. Depending on the cause, it is possible to prevent or treat them. Our authors: Ferenc Kornis and Hansgeorg Schönberger, N.U. Agrar GmbH The yield of cereals depends on the photosynthetic performance of the plants. A green leaf is a prerequisite for this. The green color of the leaf is caused by chlorophyll, which is formed in the chloroplasts of the plant cells. Nutrient deficiency, abiotic pollutants, biotic pathogens, and resistance mechanisms can attack plant cells and contribute to the breakdown of chlorophyll. As a result, leaf spots occur at these sites, reducing assimilation performance because photosynthesis no longer takes place in them. The more pronounced the spots, the more yield they can cost. Quick read Heat, fungi, pollen - there are very different causes of leaf spots, but all affect photosynthesis. But not every symptom requires plant protection measures. The wax layer protects plant cells from damage; fungal toxins, rain, or radiation can attack them. This leads to the formation of spots. Plant protection measures should not weaken the leaves further and should therefore take place in the evening. Leaf spots caused by fungi that feed on dead tissue cannot be reversed. To avoid leaf spots or react correctly to the different causes, they must be recognized and classified. a distinction is made between abiotic and biotic leaf spots. While abiotic spots occur without the involvement of living organisms, biotic spots are caused by living pathogens. This article focuses on biotic leaf spots. They are caused by viruses, fungi, and bacteria. Only the correct identification allows for targeted treatment. Greater damage from biotic leaf spots can often be averted by reacting in time and with the right measure. Viruses and bacteria Viruses, such as dwarfing viruses or mosaic viruses, cause short streak symptoms visible against the light in the leaf veins, which later necrotize and turn dark brown to black. Bacteria (Pseudomonas syringae) cause fading of husks, on which a dark ring then forms. No spore bearers are found on the bleached tissue. Fungi damage differently With fungal pathogens, it is important to distinguish between: The biotrophic obligate pathogens such as powdery mildew (Erysiphe graminis) or rusts (brown rust, yellow rust), which first form pustules and later cause yellowing of the surrounding tissue, and the necrotrophic facultative pathogens, which produce toxins after infection that destroy host cells. The fungus feeds on the dead tissue and multiplies there. While obligate pathogens can still be controlled when pustules are already visible, facultative pathogens must be eliminated before the spore-bearing structures are visible. So during the incubation period, while the tissue is still intact. Because the dead tissue can no longer transport fungicidal substances. Properly assign fungal spots Although each facultative fungus leaves different leaf spots, they can still be mistaken. Therefore, it is important to look closely: Rhynchosporium of barley starts with initial ink splatters that quickly spread apart. Within the dark, slightly jagged border, a white-gray inner courtyard with dark, often yellow-bordered spore bearers forms. In rye, initially gray-green, watery, oval spots appear on the leaf blade or in the leaf axil. Dark spore-bearing structures are visible. In severe infestations, the spots merge. Snow mold (Microdochium nivale) causes similar leaf symptoms. It also causes gray-green, watery spots on leaf blades or in the leaf axil, but with a yellowish-orange color inside. Later, the spots fade and are surrounded by a yellow edge, which can make it difficult to distinguish from Rhynchosporium. Snow mold preferably occurs on the leaf curvature. From there, the spores are washed behind the leaf sheaths, where they settle in the stem nodes. Net blotch (Drechslera teres) appears between the leaf veins as brown to black elongated spots with a yellow edge, which extend over the leaf veins (net type) and a network structure can be seen in the translucent light of the necrotic tissue. Spot types of net blotch, on the other hand, form dot or oval dark spots. They are initially mistaken for Rhynchosporium. The network structure is also visible under the microscope here. In contrast to Rhynchosporium, net blotches are permeated and surrounded by a yellow tissue caused by toxins. Rhynchosporium, on the other hand, forms a white-gray tissue within the sharply delineated dark oval ring, in which the spore-bearing structures form. Confusion with Ramularia is possible. However, Ramularia symptoms are limited by the leaf veins. Septoria tritici starts with elongated yellow spots, preferably in the upper third of the leaf. These expand and become necrotic. Inside the initially yellow-bordered necroses, the brown fruiting bodies (pycnidia) form. Septoria nodorum can also cause leaf spots, which are mainly found above the leaf sheath in the lower part of the leaf. The nodorum leaf spots are wider and less long than the tritici spots. DTR leaf blotch only occurs in wheat and susceptible triticale varieties. Mostly after wheat as a previous crop. The first symptoms appear on the lower leaves as small, dark brown spots that expand spindle-shaped and later are surrounded by a yellow edge. These cause the toxins of the fungus. Resistant varieties only react with small brown spots. On susceptible varieties, however, light brown necroses develop around the dark brown leaf spots, which spread over the entire leaf. The DTR symptoms jump from the lower leaves to the upper leaves through rain splashes. Due to its short incubation period, DTR quickly forms multiple generations. The conidial carriers and conidia are easily recognizable under the microscope and can be distinguished from Septoria species. Ramularia, now the most important barley disease in Germany, causes reddish-brown, often rectangular symptoms. They differ from leaf spots of other pathogens in the strict demarcation by the leaf veins. The spots that extend through the leaf are bordered by a yellow halo. Later, a white mold appears there. The swan-neck-shaped conidial carriers are typical. Infected leaves necrotize within a few days. Unlike the irregularly shaped net blotches, no network structure is visible inside. Powdery mildew does not make spots A special case is powdery mildew. As a biotrophic or obligate parasite, it does not kill its host plant. The formation of the fruiting body (pustule) is linked to the living host. Powdery mildew first grows punctually as a white fungal mat (mycelium) from green plant parts and then spreads over the leaf. The mycelium can be wiped off with a finger. With persistent infestation, the surrounding tissue of the fungus lightens and becomes chlorotic because the powdery mildew sucks assimilates from neighboring cells. On the underside of the leaf under the powdery mildew pustules, the tissue remains green for longer. Cytokinins are enriched there. If the leaf later dies, the green tissue under the old pustule darkens. The cereal itself develops its own defense spots against powdery mildew. Powdery mildew-resistant varieties or age resistance can starve powdery mildew by enriching phenols. Instead of the mycelium, sharply demarcated, black-brown leaf spots (formerly tar spots or Theresa spots) appear, which also fall under the term mlo spots. Under the microscope, often rudimentary remnants of the now grayish powdery mildew mat can still be seen. Plant protection: Avoid spray spots The cuticle and wax layer protect plant cells from environmental influences such as radiation, moisture, or heat. Fungi or pests also have to penetrate this barrier before they can damage the plant. The same applies to plant protection products. To work inside the plant, they must first overcome the wax layer and cuticle. Sharply formulated agents attack the wax layer. This limits protection against UV radiation. Because water droplets linger longer and act like a magnifying glass in the sun. Thus, light rays can penetrate concentrated into the upper cell layers of the leaf and cause burns. The plant tries to defend itself, produces increased ethylene, and concentrates it in the area of the damaged cells. These then die and appear as yellow points, which become larger and later necrotic. These symptoms are less pronounced after plant protection applications in the shade. The following circumstances contribute to impairing the wax layer: A highly concentrated spray droplet, if it does not spread on the leaf, especially where tissue weakened by nutrient deficiency produces less wax. Therefore, symptoms occur more frequently in poorly nutrient-supplied areas in the field. A pathogen has already penetrated the leaf and wax formation is interrupted (powdery mildew, rusts). Here, often punctate spots are formed. When facultative parasites produce toxins. Even before initial yellowing is visible, wax formation is stopped. The spray solution then attacks the wax layer more strongly at these points. Often, symptoms similar to Septoria, DTR, or yellow rust are only caused by the spraying, but no spore-bearing structures are visible. The following measures can help prevent spray spots: Do not spray immediately after a (cool) rainy period. Two to three days of sun are needed for the wax layer to reform. Avoid sharply formulated plant protection products that attack the wax layer. Also, active ingredients that quickly penetrate the leaf (e.g., morpholines) and impair the wax layer (Moddus, EC-formulated insecticides). If these agents are absolutely necessary, choose the lowest application rate and preferably spray at night. Postpone spraying to the evening and night to prevent damage from direct radiation. Add spreaders (e.g., Break thru, Silwet Gold, Axcess) that distribute the spray droplets as a film on the leaf. This avoids the locally elevated concentration under the spray droplet.