Heat and Rains Result in Corn's Common Rust

Recently plant pathologists discovered the heat and recent rains make for corn's common rust which are gray leaf spot and physoderma brown spot. They are starting to surface across the Corn Belt, and a relatively new disease, bacterial leaf streak of corn, is spreading rapidly in the Western states, as well. After scattered and sometimes severe hail storms battered many Midwest cornfields without anti hail net in recent weeks.

Gray leaf spot thrives in hot, humid environments, so it's bound to love the recent soaring temperatures and damp fields where he lives. Which Means that growers face a fairly tight window for spraying, with the ideal time falling between VT and R1. Earlier sprays around V7 or V8 might not last long enough to control late-season disease pressure. Many of the best fungicides are only going to last three weeks. Meanwhile watch the lower leaves, because gray leaf spot usually starts at the bottom

The disease's gray or tan lesions will be rectangular in shape and run tidily between leaf veins, with smooth edges. That characteristic can help you distinguish them from a new lookalike disease -- bacterial leaf streak. The gray leaf spot is well controlled by fungicides and ground cover, but it's important to note which hybrids are succumbing to the others. For many of these, hybrid selection is going to be the most effective and economical management strategy. 

Physoderma brown spot is caused by a "fungus-like" pathogen with swimming spores. The spores often infect the plant through water-filled corn whorls. As the whorls fill up and dry out over the weeks, the spores create a banding pattern on the leaves. The infection produces two types of lesions. The first are small, bright-yellow dots, scattered across the leaf and often mistaken for southern corn rust. Remember that southern rust pustules will rub off -- these lesions will not budge.

The second type of lesion are larger and black and occur only in the midrib of the leaf. Physoderma brown spot lesions can also infect the nodes and make the stalk brittle and vulnerable to lodging and breaking, if you find this you can use plant support net so consider marking severely infested fields for early harvest.