Farmers in the U.S. are increasingly running into unfamiliar signs of crop diseases with changing climate patterns allowing pathogens to take root in new areas. As correctly identifying the problem is one of the most crucial steps in preserving crop yields, we put together some information on disease threats being reported in new regions across key growing regions.
Corn:
Tar Spot – This relatively new disease is caused by two fungi that affect ear husks and leaves. More prevalent in northern counties, it can come in later. Tar spot overwinters in residue, so if it has been identified in your area before, inoculum is likely around. This disease takes hold fastest when it’s cool and humid with prolonged leaf wetness. Even if it’s a dry season, if it turns wet later, be on the lookout for tar spot. It starts out as tiny black specks and can take over whole plants quickly if weather conditions are favorable.
Southern Rust – This fungus doesn’t typically survive in Corn Belt climates over winter and must blow in from Southern states. Its presence was reported in some areas in 2020. Typically, it can start showing up from July forward. Dew periods of at least seven hours favor rust development, according to observations by Purdue specialists. The folks over at Farm Progress note that people at one time believed southern rust came in too late to do much harm in the Corn Belt. However, just a few seasons back, it came in hard during the first weekend of September on a storm front and zapped later-planted corn. Some people chose not to spray because it was late, and they didn’t expect big yields anyway. In retrospect, that was a mistake because yield losses were often large. Southern rust is caused by the fungal pathogen Puccinia polysora and is generally more damaging to corn than common rust due to its ability to rapidly develop and spread under favorable conditions. Southern Rust Has small circular, pinhead-shaped pustules that are reddish orange in color. It only infects the upper leaf surface, where as common rust infects both upper and lower leaf surfaces.
Bacterial Leaf Streak – Bacterial leaf streak is caused by the bacterium Xanthomonas vasicola, and has been observed on field corn, seed corn, popcorn and sweet corn. Symptoms begin as narrow leaf lesions with wavy edges that occur between the veins of corn leaves and can range between one-to-several inches long. Lesions can be brown, orange, and/or yellow and are often yellow when backlit. This emerging disease had only been reported on corn in South Africa up until 2016 when it was first confirmed in Nebraska. The pathogen survives in infected corn debris from previous seasons and is thought to infect the plant through natural openings in the leaves. Irrigation and wind-driven rain, as well as warm temperatures, are thought to exacerbate the disease. Foliar fungicides used to manage gray leaf spot and other fungal diseases are not expected to effectively control this bacterial pathogen.
Soybeans:
Taproot Decline – Soybean taproot decline is caused by a group of fungi called Xylaria — which are normally a wood-rotting family of pathogens. But scientists officially confirmed that this fungal group was causing soybean root and stem rot in 2017. The fungus first infects the roots of a soybean plant and its symptoms can surface at any time during the growing season, according to the Illinois Soybean Association. It can cause wilting plants, interveinal necrosis, plant death and spotty stands from poor emergence. Taproot decline has a distinctively scattered pattern in fields, with healthy plants sitting alongside affected ones. To diagnose taproot decline in the field, growers must split the stem. Like SDS, the inner pith will be white, but the roots will be a striking jet black, with very few fine roots attached. Fingerlike projections that sometimes appear at the base of the stem are fungal structures called stromata. The fungus overwinters in the soil and scientists have found that precision planting practices that allow farmers to place soybean seeds in exactly the same slot year after year, can increase infection rates.
Sudden Death Syndrome – More commonly known as SDS, this disease is caused by the soilborn fungus Fusarium virguliforme that attacks the roots of soybean plants and produces a toxin in the root that moves up into the leaves. SDS has two phases–a root rot phase and a leaf scorch phase. The fungus is closely related to another soybean pathogen, Fusarium solani form B, which causes seedling disease and root rot. The disease’s importance varies greatly throughout production areas. In some years the disease may be present in a high percentage of fields across an entire state and in others it can be localized or rare. SDS has clearly been moving further north with states like Minnesota and North Dakota now at risk. The foliar phase of the disease produces symptoms that can be confused with other diseases, most notably stem canker and brown stem rot. Phytotoxicity symptoms from some triazole fungicides also can be easily confused with those of SDS. It is well documented that there is a high degree of association between the presence of soybean cyst nematode (SCN) in a field and SDS development.
Red Crown Rot – Red crown rot in soybeans is caused by a fungus called Cylindrocladium crotalariae, which infects roots early in the season after planting, but plays the long game. Symptoms often don’t emerge until R3 to R5 growth stages, at which point, it can cause interveinal chlorosis, wilting, dying plants and rotting roots. As the name suggests, red crown rot is distinguished by the tiny red structures called perithecia that cover the stem surface near the soil, giving it a reddish tinge. When split, stems centers will have a gray look kin the lower part of the crown root rather than healthy white pith. The disease does well in warm, wet soils and it can overwinter and survive for two to three years in the absence of a soybean host. As a result, management recommendations mostly involve rotating away from legumes for at least two years, avoiding heavy residue and improving drainage in wet spots in the field.
Cercospora Leaf Blight – This crop disease has long been a problem in the south but has recently been pushing further north into central regions of the U.S., according to the Soybean Research and Information Network. Cercospora leaf blight is caused by a fungus that creates discoloration on leaves’ surface with light purple irregularly shaped patches. As the disease worsens, the leaves fall off the plant. Unlike other diseases, it rarely causes yield loss unless untreated. The disease favors warm and humid conditions.
Brown Stem Rot and Frogeye Leaf Spot – Two other diseases newly threatening northern states. Brown stem rot is fostered by cool temperatures during flowering and pod fill, and also can be boosted by the presence of soybean cyst nematode. BSR, as it’s commonly called, can cause yield losses of more than 30%, though losses in the 10-20% range are more common. Symptoms often aren’t visible on leaves, and plant stems must be split to diagnose the disease. Frogeye leaf spot is recognized by brown/tan spots surrounded by a brown, purplish ring on leaves. The spots can grow together and kill large areas of leaves. It does best in warm, humid weather, and can be managed with tillage, crop rotation and fungicides. (Source: AgFax, Missouri Independent, Farm Progress, UNL, IA State, Soybean Research Info Network)
