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It rained. Be ready for rustBy Miles Dabovich Texas AgriLife Extension agent, agriculture, Wichita County Texas Wheat producers in Southwest Oklahoma and North Texas finally find themselves in a unusual, possibly unprecedented, situation. Grain prices are at an all time high and recent rains have now taken us one step closer to a successful harvest. The prospect of a profitable harvest makes it imperative to protect our potential yield. Most of the inputs necessary to insure a good harvest are behind us. Fall fertilizer and spring top dress applications have been made, herbicide should already have been applied and cattle have been removed from fields to be harvested for grain. Later planted wheat or wheat that emerged late due to drought still needs to be monitored for greenbug activity but most wheat is old enough to withstand relatively high numbers of greenbugs and beneficial insects are increasing with increasing temperatures. So what do we worry about now? Wheat rust! The two diseases that most radically effect our annual wheat production are leaf rust and stripe rust. Both are fungal diseases that become prevalent in any year that is favorable for wheat. In other words, if it is a good year for wheat grain yields, it generally also is a good year for rust. Rust spores are carried on wind currents from south Texas and germinate on wheat leaves when there is free moisture such as dew or rainfall. South Texas has been in a more extended drought than the wheat areas further north so rust development has been significantly delayed. Rust is now appearing in fields in central and south Texas so the potential for an epidemic is not past. Stripe rust has the potential to be devastating if it occurs early. This is a relatively new disease in the Southern Great Plains but it has limited yields in various areas of Texas and Oklahoma every year since 2001. Stripe rust can be identified by linear arrangement of yellow spore masses on the leaves. If a stripe rust infection begins anytime after jointing and if environmental conditions are conducive to fungal growth, grain yield can be a total loss. Ideal conditions for the disease pathogen are cool temperatures and high humidity. Leaf rust is the fungal disease most familiar to wheat producers. It is easily distinguished from stripe rust because its spore masses occur randomly along the leaf surface rather than linearly and because the spore masses are red rather than yellow. We generally have light levels of leaf rust throughout the fall and winter. Whether or not an epidemic develops, again, depends on weather. Unlike stripe rust, leaf rust prefers warmer t temperatures but, like strip rust, it needs high moisture for its spores to germinate and infect the plant. There are several other diseases that can infect Southern Plains wheat producers. Many of these diseases previously were considered minor but have become more common as we increase our use of conservation tillage practices. Powdery mildew has been severe in the past two years primarily due to higher acreages planted to susceptible varieties. Mildew can occur at any time during the season but is more severe when temperatures are low and humidity is high. It is characterized by fluffy white or gray spore masses on leaves or stems and, when severe, can weaken the stem and cause lodging. Septoria leaf blotch and tan spot are two diseases that can effect grain yields primarily by decreasing the area of the leaf capable of conducting photosynthesis. Control measures for all these diseases are similar. Resistant varieties are the first line of defense. There are several recently released varieties with high levels of resistance to the two rusts. Both rusts also can be controlled with fungicides and, with high wheat prices, The use of fungicides on susceptible varieties is more practical than ever before. Timing of the fungicide application is important. The critical factor in the use of a fungicide is to protect the functionality of the flag leaf. The most effective fungicides are systemic but their systemic activity only persists for a restricted period. In order to keep the flag leaf functional through the majority of the grain filling period, it is best to delay fungicide application until viable spore masses are visible on the flag leaf. At that time, speed is of the essence and the crop should be sprayed as soon as it is practical. Recent rains are a God-send to the regions wheat production potential and make it increasingly important to remain vigilant and protect what the crop currently has to offer. For further information on wheat varieties and appropriate cultural practices, contact me at 940-716-8610. 3/31/08 Date: 3/27/08
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