MU plant pathologist gives update on emerging crop diseases

New University of Missouri Extension plant pathologist Kaitlyn Bissonnette urges growers to notify her if they spot southern rust and other diseases this growing season. She gave growers a look at emerging diseases at the second annual NEMO Soils and Crop Conference recently in Palmyra.

Corn producers reported southern rust in parts of Missouri in 2017 and Bissonnette suspects it is more prevalent than reported. Southern rust appears on leaves as small, circular, reddish-orange pustules. It grows best in warm weather and under high relative humidity. It spreads rapidly and can cause yield loss if infection starts around tasseling. Its spores blow in from southern states and Mexico.

Southern rust shows in later-planted corn, so Bissonnette recommends timely planting and treatment with fungicides if infection begins early, especially under irrigated systems.

Other diseases to look for include:

Gray leaf spot—It thrives in 90 percent or higher humidity and can be identified by the rectangular lesions that develop on leaves. During the season, these lesions produce spores that can continue to infect healthy tissue. Rotate crops and choose resistant hybrids to prevent this disease, she says.

Sudden death syndrome in soybean—Yield loss from this fungal disease is generally not uniform, as SDS begins by appearing in patches or “hot spots” in fields. It occurs most often in cool, wet soils such as early-planted and compacted fields. The fungus stays in soils and infects soybean roots soon after seed germination. Symptoms of this disease, which develop during the late vegetative or early reproductive stages, are characterized by interveinal yellowing and tissue death of the leaves.

Bissonnette says growers can distinguish SDS from other soybean diseases by looking at roots and the inside of stems. In later stages of development, infected roots can develop blue fungal structures on the outside of the stem at the soil line. These are the spore-producing fruiting bodies of the fungus. Cut and examine the inside of soybean stems. While brown stem rot is characterized by the brown color of the pith tissue, the pith of plants infected with the SDS pathogen will appear normal, with adjacent tissues having only a slight brown discoloration compared to a healthy stem. To reduce severity, maintain good seedbed conditions. Avoid soil compaction and planting too early in the season when soils are cool and wet, Bissonnette says. Foliar fungicides are not effective on SDS.

Soybean cyst nematode—Bissonnette says 75 percent of Missouri fields contain SCN, a parasitic worm that penetrates roots and reduces water and nutrient uptake. It favors dry, sandy soils. In Missouri, SCN can produce anywhere between three and six generations in a growing season. SCN does not always produce symptoms. Soil testing is the only way to know if it is present. SCN can be difficult, but not impossible, to manage, Bissonnette says. Choose SCN-resistant varieties and rotate to non-host crops.

Fusarium head blight of wheat—Also called scab, this fungal disease results in shriveled, lightweight wheat kernels and the accumulation in the grain of a mycotoxin called DON or vomitoxin. Bissonnette recommends planting resistant cultivars, applying fungicide at 50 percent flowering when the risk of infection is high, and rotating crops. The pathogen is associated with Gibberella ear rot and Gibberella stalk rot of corn, so avoid planting wheat into infested corn residues.

Goss’s wilt—Bissonnette sees this bacterial disease as an emerging problem in Missouri. Infected corn plants exhibit dark freckles on the leaves that develop into elongated lesions. It often appears in fields when hail wounds plants, leaving openings for the bacteria to enter. The pathogen is associated with corn residue and is more severe in fields continuously planted to corn under reduced tillage. Plant resistant hybrids, rotate crops, control weeds and manage residue to avoid Goss’s wilt. It is easily confused with other diseases and difficult to diagnose, Bissonnette says. She recommends submitting samples to the MU Plant Diagnostic Clinic at plantclinic.missouri.edu.

Bacterial leaf streak—First reported in Nebraska in 2016, this pathogen spreads by wind and often follows hail damage. It appears on corn leaves as brown or tan lesions surrounded by yellow halos. It is often confused with gray leaf spot. It likely overwinters in crop residue, so Bissonnette recommends crop rotation. This disease has not yet been reported in Missouri, she says, but growers should be on the lookout. She recommends submitting suspected BLS samples to the MU Plant Diagnostic Clinic for confirmation.

Sclerotinia stem rot—This fungal disease appeared in northwestern Missouri in 2017 following cool, wet weather. Bissonnette says there is a low likelihood of it establishing in northeastern Missouri.

To reduce disease risk, Bissonnette recommends that growers rotate crops, plant resistant varieties when available, and scout early and often. For SCN, soil tests provide better accuracy than visual inspections, so “take the test to beat the pest.”

For more information, contact Bissonnette at [email protected] or follow her on Twitter at @kmbiss.