Population Dynamics of Wheat Curl Mite with Deficit Irrigation and Cultivar Mixtures

ABSTRACT Wheat streak mosaic virus (WSMV), vectored by the wheat curl mite, Aceria tosichella Keifer, (WCM) is one of the major limiting factors in production of wheat (Triticum aestivum L.) in the Great Plains region of the United States. In addition to its direct effect on yield, the virus significantly reduces water-use efficiency, which is of concern in semi-arid regions such as the Texas Panhandle where irrigation water is limited. Severity of wheat streak has been observed to increase during years of severe drought, possibly because of greater abundance of mites. Other mite species also have been reported to increase in numbers on drought-stressed plants. In the Texas Panhandle, much of the wheat crop is irrigated, but because of depletion of the water in the Ogallala Aquifer, less than the full amount of water required for production of crops often is applied. Over time wheat lines have been bred to tolerate drought conditions in areas with scarce rainfall. However, little is known of the optimal reproductive capability of wheat curl mite under deficit irrigation in a wheat field and its impact related to disease severity. Therefore, field studies were done using two wheat cultivars, Karl 92 (susceptible) and TAM 112 (tolerant), at three amounts of irrigation (33, 67, and 100% of potential evapotranspiration rate) at the Texas A&M AgriLife Research Station, at Bushland, TX, in 2013 and 2014. Numbers of mites, severity of disease and soil moisture content were evaluated for each cultivar and amount of irrigation water applied. Overall, mite numbers increased as irrigation amounts decreased (R² = 0.5077, P = 0.0009). Also, there was a slight but significant positive correlation between severity of disease and soil moisture (R² = 0.3307, P = 0.0125). Studies in a greenhouse assessed abundance of wheat curl mite and incidence of wheat streak mosaic. More wheat curl mites were found in the 33 and 67% water-replacement treatments for the TAM 112 cultivar. These results showed that under deficit irrigation, the wheat curl mites increased in abundance, which in turn resulted in increased disease incidence and reduced water use-efficiency, leaving unused water in the soil. Irrigation of mite-infested wheat can result in wasted water in regions where water is limited.
In a second study, mixtures of TAM 112 and TAM 111 were evaluated for their impact on incidence of WSM and WCM. Initially bred for tolerance to drought, TAM 112 has shown resistance to wheat curl mite and tolerance to Wheat streak mosaic virus under drought conditions in the field. Because TAM 112 is the only drought-tolerant cultivar in the Great Plains region, with recognized resistance to WCM and WSMV, new methods of control are essential. Use of wheat mixtures with cultivars TAM 112 for resistance to WCM and TAM 111 for resistance to rust, offers promise to maximize yields in the presence of both the mite and leaf rust. Therefore, five mixtures of TAM 111 (susceptible) and TAM 112 (tolerant) wheat (25% TAM 112: 75% TAM 111; 50% TAM 112 and TAM 111; 75% TAM 112: 25% TAM 111; 100% TAM 111; and 100% TAM 112) were evaluated in 2013 and 2014 in fields at the Texas A&M AgriLife Research Station, Bushland, TX, for their impact on the spread of WCM and incidence of disease. Number of mites and incidence of disease were evaluated among cultivar mixtures in the field. Numbers of WCM increased over time in response to warmer spring temperatures (P < 0.0001), with a significant decrease in WCM at distances farther from the point of infestation (P < 0.0001). Results from greenhouse studies, in conjunction with the field studies also revealed a significant increase in mite numbers over time as well (P < 0.0001). The number of mites over distance and time were significantly less on 100% TAM 112, with a noticeable trend in increasing numbers of mites within decreasing percentages of this resistant variety. These results showed that TAM 112 can limit the spread of wheat curl mite throughout the field and has potential to do so in a cultivar mixture.