Agriculture & Natural Resources: Corn disease scouting
Sources: Dr. Kiersten Wise; UK Plant Pathologist As a continuum from last week, other corn diseases have been present this year with our high heat and humidity. Here are a few more diseases that are common this year in addition to Gray Leaf Spot that we learned about last week.
Diplodia leaf streak, caused by the fungus Stenocarpella macrospora, can be confused with gray leaf spot in the early stages of development. Small, elongated lesions appear on leaves, sometimes in the mid-canopy. The location of these lesions can help distinguish it from gray leaf spot, which typically appears in the lower canopy and progresses into the mid-upper canopy. Leaf streak lesions will expand over time into streaks that are several inches or more long. Small black fungal structures may be visible in the center of the elongated lesions. Diplodia leaf streak is a disease that has recently become more prominent in corn, and the link between disease and yield loss has not been established. Currently, there are no fungicides labeled for Diplodia leaf streak. The fungus that causes Diplodia leaf streak survives in residue and therefore rotation or residue management can help reduce disease in future years.
Holcus leaf spot is very common in Kentucky corn this year. Holcus leaf spot is a disease caused by the bacterium Pseudomonas syringae pv. syringae.
It is characterized by round, discrete lesions that are initially pale yellow to white and then enlarge and turn gray or brown. Lesions have a water-soaked halo and, on certain hybrids, the margin of the lesion may appear brown or purple.
Holcus leaf spot is not known to limit yield. Although the disease may cause concern based on symptom appearance, no in-season treatment is available or necessary. Fungicide applications will not have efficacy against this bacterial disease.
More information on holcus leaf spot can be found in the UK Extension publication, Holcus Leaf Spot (PPFS-AG-C-06).
Physoderma brown spot is caused by the fungus Physoderma maydis, which also survives in soil and residue. Corn plants can become infected when plants are ponded or excess water remains in the whorl. The symptoms typically appear in the mid-late vegetative stages through pollination and are characterized by very small chocolate brown or yellow lesions on the leaves and midrib. The lesions may appear in a banded pattern and can also be found on the stalk, leaf sheath, or ear husks. Physoderma leaf spot rarely needs management and is usually only problematic in wet springs. Improving soil drainage and removing infected plants will reduce the disease risk for subsequent crops. Fungicides are labeled for Physoderma brown spot management, but symptoms are usually not severe enough to warrant fungicide applications.
More information on Physoderma brown spot can be found in the UK Extension publication, Physoderma Brown Spot (PPFS-AG-C-07).