MU Plant Diagnostic Clinic: Woes of Wheat and Forage, 2016

Published: March 2, 2017

Corn and soybean are most often submitted to the Plant Diagnostic Clinic, but occasionally we also receive wheat and forage crops. In 2016, 18 samples of wheat, 5 samples of alfalfa and 4 samples of fescue were processed through the lab.

Wheat samples included foliar diseases that are routinely diagnosed, including bacterial leaf streak, Septoria leaf blotch and rust diseases. Rust diseases were quite notable and severe in 2016, with both leaf and stripe rust in abundance. Both types of rust were often observed on the same leaf (Picture 1). Differentiation of the two rust diseases is observed in pustule development, spore coloration and spore morphology. Also, in wheat, both wheat streak mosaic virus (WSMV) and barley yellow dwarf virus (BYDV) were diagnosed. The plant clinic offers a 5 virus screen that includes WSMV, BYDV, wheat spindle streak mosaic virus (WSSMV), soil-borne wheat mosaic virus (SBWMV) and cereal yellow dwarf virus (CYDV), for an additional service fee of $20. Besides foliar issues, we also diagnosed Sharp eyespot on two samples. This disease causes an elliptical lesion on the lower stem (Picture 2). Within symptomatic leaf sheath cells, right-angled mycelium is observed indicative of the fungal pathogen Rhizoctonia cerealis.

Picture 1: (left) Wheat leaf with both leaf and stripe rust. Leaf rust is darker in color and the pustules are scattered on the leaf. Stripe rust is more yellow and the pustules are in stripes. Picture by Patti Hosack. Picture 2: (right) Sharp eyespot lesion on a sample of wheat. Picture by Patti Hosack.

Picture 3: (left) A sample of alfalfa with violet root rot, note the dark purple mycelium covering the tap roots. Picture by Patti Hosack. Picture 4: (right) Field symptoms of violet root rot include large patches with dead plants at the margins. Often a disease resistant grass species (such as fescue) grows in the voids. Picture by Wyatt Miller.

Picture 5: Alfalfa with summer black stem and leaf spot. Symptoms include black spots on leaves, chlorotic leaves and defoliation. Picture by Patti Hosack.

Picture 6: Cercospora medicaginis spores observed in the black lesions on leaves and stems of alfalfa, diagnostic of summer black stem and leaf spot. Picture by Patti Hosack.

An alfalfa sample won the 'disease of the year award', with the criteria being impressive field symptoms and distinct pathogen signs. In the fall, a sample was mailed in with violet root rot. This disease is caused by the soil-borne fungus Helicobasidium purpureum (syn. Rhizoctonia crocorum). As the species name implies, the roots and crown of the plant are covered with dark purple mycelium (Picture 3). Field symptoms were striking - large patches with dead or dying plants around the margin and disease resistant fescue growing in the center (Picture 4). Also diagnosed from two alfalfa samples was summer black stem and leaf spot caused by the fungal pathogen, Cercospora medicaginis. This disease causes black spots (lesions) on leaves, leaf chlorosis, defoliation and black stem lesions (Picture 5). Copious amounts of pathogen spores are observed within the lesions and allow for identification (Picture 6). Abundant summer rainfall and high humidity in July and August spurred on black stem and leaf spot. Insect damage to alfalfa foliage, most caused by potato leafhoppers, was also observed on two samples.

Forage fescue wraps up the miscellaneous agronomic crops. The only disease diagnosed was common leaf rust caused by Puccinia spp.