The heat is on and I believe I am ready for snow. Maybe not to that extreme, but I am ready for a cool down. The rain this week was welcomed by all grain producers but it would be nice if we could continue getting some each week.
OSU Entomologists remind producers that with continued hot, mostly dry weather it is important to remain vigilant for emerging twospotted spider mite problems in field crops. Look for characteristic yellow stippling on leaves and confirm the presence of mites by tapping vegetation over a piece of black construction paper (which works better than white paper, though white will do) and looking for dust that crawls.
Increasing mite populations often start on field edges, and edge treatments may work if problems are caught early. But if mites are found beyond the field borders and if conditions are very favorable for mite increase (continued hot, dry weather with low chance of rain), either make your treatment decision for the whole field, or be prepared to scout often and vigilantly and treat quickly when interior populations increase.
Threshold recommendations for spider mites in soybean are summarized in a previous newsletter article:
http://agcrops.osu.edu/newsletter/corn-newsletter/insect-pests-we%E2%80%99re-watching-now
Peter Thomison at Ohio State have noticed especially in northern corn fields exhibiting drought and temperature stress. Leaf rolling is common and uneven, stunted corn growth is widespread. Moreover, most of these stressed corn fields are at a stage of development that is especially susceptible to drought and high temperature injury – pollination.
High temperature stress is usually associated with drought. Heat stress and drought intensify damage to corn and soybean but either may cause major crop injury alone. The recent high temperatures have generated questions about the impact of high temperatures on corn. How much heat stress can these crops withstand without incurring major yield losses even when adequate soil moisture is available?
Corn originated as a tropical grass and can tolerate exposures to adverse temperatures as high as 112 degrees F for brief periods. Optimal daytime temperatures for corn typically range between 77 degrees F and 91 degrees. Growth decreases when temperatures exceed 95 degrees F.
University of Illinois agronomist Dr. Emerson Nafziger notes that “afternoon temperatures in the mid-90s are not a problem for corn … if they have enough soil water available … plant temperatures have been raised to 110 or higher without doing direct damage to photosynthetic capacity. The level required to damage leaves depends on the temperature the leaf has experienced before, but it generally takes temperatures above 100 in field-grown plants”
According to Iowa State University agronomists, high temperatures may have a double impact on corn. The first is the increase in rolling of corn leaves in response to moisture deficiency. By rule-of-thumb, the yield is diminished by 1 percent for every 12 hours of leaf rolling — except during the week of silking when the yield is cut 1 percent per 4 hours of leaf rolling.
The second impact is less obvious initially. When soil moisture is sufficient, as it is for the most part this July, the crop does not have a measurable yield response to one day of temperatures between 93F to 98 F. However, the fourth consecutive day with a maximum temperature of 93F or above results in a 1 percent yield loss in addition to that computed from the leaf rolling. The fifth, day there is an additional 2 percent loss; the sixth day an additional 4 percent loss. Data are not sufficient to make generalizations for a heat wave of more than six days, however firing of leaves then becomes likely and very large yield losses are incurred.
Generally a six-day heat wave at silking time is sufficient to assure a yield not to exceed trend. Some corn agronomists believe newer corn hybrids may have more tolerance to heat and drought than the hybrids did when this rule of thumb was originated.
Are corn hybrids that show greater leaf rolling more susceptible to drought injury than other less affected hybrids? Not necessarily. In a recent OSU evaluation comparing two drought-tolerant and two drought-sensitive hybrids, differences in leaf rolling were not readily evident.
However, they were observed at Hoytville, OH in 2012, a site-year that experienced severe drought stress. Leaf rolling ratings for the drought tolerant hybrids were lower or about the same as the drought sensitive hybrids.
However, other types of drought tolerant hybrids may express different leaf rolling responses.
Tony Nye is the state coordinator for the Ohio State University Extension Small Farm Program and has been an OSU Extension Educator for agriculture and natural resources for 28 years, currently serving Clinton County and the Miami Valley EERA.
