Rain Impact on Crops and Disease
Rain this past week caused some flooding, especially in the several counties around Lima, Ohio and further south and west. Rainfall amounts of 9-10 inches were recorded. Most of Northwest Ohio got several rainfall events over several days. Officially, the 5-year drought appears to be over.
Flooding effects on corn and soybean development varies and depends on crop stage of development, soil types, and how long water remains in the field. For the State of Ohio, if the corn was planted early and the corn is more mature (dough to dented), the flooding effects may be less severe; unless the corn stands in water for extended period of time (past 2 days).
For northwest Ohio, delayed spring planting delayed corn growth development. Corn has caught up somewhat but a lot of corn is just past the tasseling stage and is setting kernels. This can be a tough stage for corn development if fields remain flooded for extended periods of time. Some corn fields may have had high winds, lodging and even green snap which causes corn to break off. Root lodging from R2-R4 when corn is developing kernels may result in 20% to 40% loss in grain yield. Often this creates harvesting challenges with down ears and poor grain quality due to less air movement on downed ears. Increased incidences of corn ear diseases may result.
Ohio State University researchers report, “If corn plants have been bent or snapped, yield losses may be incurred. Stem buckling during the reproductive stages may cause 15-20% yield loss, and for every 1% of plants that green snapped (full stalk breakage below the ear), a 0.5-1.0% loss in yield can be expected. Corn yield losses from flooding during grain fill stages tend to be much lower, with yield losses being 5-7% for flooding up to 3 days, or around 15% if flooding lasts for 6-7 days at the R1-R2 growth stage.” (Lindsey, Lindsey, Ortez).
For soybeans, flooding can be more detrimental especially during flowering and seed set. Again, it depends on variety and how long water stands or ponds in the field with saturated roots. Yield losses can range from 15% to 90% or more if the plants are dead or dying. Be sure to keep an eye on your fields to see how quickly the flooding or saturated conditions remain as this will likely affect the magnitude of any potential yield loss. Yield losses are higher the longer water stands or ponds in a field. Other issues include muddy and sediment covered leaves which candisrupt or prevent proper photosynthesis from occurring. Light showers after a crop gets mud soaked may be beneficial.
One corn disease that likes humid and moist conditions is Gray leaf spot (Cercospora zeae-maydis, GLS). The disease usually starts out on the lower leaves from rain and muddy water splashed up on the leaves. It creates narrow rectangular leave lesions that are parallel to the leaf. Yield losses can range from 5 to 50 bushel per acre. It thrives during warm humid weather. If GLS infects the ear leaf, that is where about 80% of the energy from photosynthesis is used to develop that ear of corn and can be highly detrimental to corn yields.
The GLS lesions appear tan to brown in color and rectangular in shape, bordered by the veins of the leaf. Individual lesions may be 3 to 4 inches long and 1/16 to 1/8 inch wide, depending on the distance between veins. The light brown lesions produce conidia (spores) and the affected area turns a silvery-gray color. Some corn hybrids may have orange or yellow lesions and some may develop symptoms on the stalk and leaf sheath. The more area of the leaf that is covered with lesions, the more the yield damage which increases with time.
Currently, GLS resistant hybrids have been developed. However, in the 1990’s, before GLS resistant corn, yield losses were 90-100%. GLS rarely reaches that extreme today. If GLS is below the ear leaf, grain damage is usually minimal, however; if it get above the ear leaf, expect much more damage. Fungicides are often applied to prevent GLS disease. Watch for susceptible varieties or varieties where corn is planted into a lot of residue. High GLS disease often occurs in low areas and river bottoms where high humidity and low air flows occur.
GLS overwinters on surface corn residue. In late spring, warm temperatures with high humidity, produces conidia (spores) which can infect lower corn leaves in June to late July. Late rains increase infection in late July and August from wind-blown spores. The earlier GLS infection occurs; expect more leaf and yield damage.