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Aphids and Disease Spread in Crops

Mixed cropping is a cultural control method that can help protect plants from certain aphid-borne virus diseases. Soybean mosaic virus (SMV) and maize dwarf mosaic virus (MDMV) are transmitted in a nonpersistent manner by several species of winged aphids. The transmission of these viruses is termed "nonpersistent" because the virus is both acquired and transmitted in a matter of seconds by a large variety of aphid species that subsequently lose their "charge" or ability to transmit these viruses within an hour. SMV is found wherever soybeans are grown and may cause severe yield losses if early-season spread occurs. Found particularly in areas where johnsongrass occurs, MDMV is one of the most important diseases of sweet corn, resulting in serious yield loss when young plants become infected.

Researcher Gwen Fondufe weeding soybeans and corn. Photo by Gail Kampmeier.

To spread, viruses such as SMV and MDMV rely primarily on transient aphid vectors moving through a field. One factor that governs the rate of virus epidemics is the interaction of the aphid vectors with their environment. The overall pattern of vegetation in a field may influence aphid take-off, flight, and landing activity. This includes plant density, canopy cover, crop height and architecture, host plant suitability, and the presence of barrier crops. These relationships are often complex and differ with various aphid species, necessitating a thorough understanding of specific crop/pest interactions when trying to predict the influence of intercropping on a particular pest or disease system.

Companion crops, particularly plants that are not hosts of the virus, effectively slow down the spread of aphid-borne virus diseases. Aphids carrying nonpersistently transmitted viruses tend to lose their infectivity upon probing a nonhost of the virus. It is therefore reasonable to assume that the higher the proportion of nonhost plants of the virus in a crop mixture, the greater the probability that an aphid landing and probing on a nonhost plant will reduce or lose its infectivity, resulting in a proportional reduction of virus incidence.

In a two-year field study, soybean (a nonhost of MDMV) was intercropped in different proportions with sorghum in 1990 and corn in 1991, both nonhosts of SMV. The focus of this study was to understand the influence of varying soybean/cereal mixtures on the incidence of aphid-borne nonpersistently transmitted plant viruses, such as SMV and MDMV, and also on the landing rates of the aphid vectors that transmit them. By identifying the factors underlying virus spread, this study would contribute to the small arsenal of tactics for managing virus diseases and their vectors.

The experiments were conducted in Champaign, IL, during the summers of 1990 and 1991. Four replicates of five different proportions of soybeans and corn or sorghum were planted: (1) soybean monoculture; (2) two rows of soybean to one row of sorghum or corn; (3) one row of soybean to one row of sorghum or corn; (4) one row of soybean to two rows of sorghum or corn; and (5) sorghum (1990) or corn (1991) monoculture.

Aphid landing rates, which give a measure of vector activity within a plot, were monitored with liquid-filled pan traps consisting of a green tile in a plastic sandwich box that was maintained at the crop canopy level. Aphids were collected daily and identified to species under a microscope. Four of the most common species of aphids found during the summer in Illinois field crops, including the corn leaf and melon aphids, were highlighted in this study. Portions of each plot were initially inoculated with SMV or MDMV by airbrushing or rubbing the leaf surface with a mixture of infected plant sap and an abrasive. Natural spread of these viruses by aphids was monitored weekly throughout the growing season by examining plants for symptoms.

Somewhat surprisingly, given that soybeans are not hosts of any species of aphid in the U.S., more aphids landed in soybeans than in either corn or sorghum. In soybeans, landing rates were generally higher in the mixtures than in the monocultures. Sorghum and corn plants were taller than soybeans throughout the growing season and may have served as barriers, protecting aphids at the soybean canopy from the greater turbulence and higher wind speeds usually experienced in the open monoculture. This protected environment may have encouraged greater flight activity in the crop mixtures. In sorghum and corn, landing rates in the mixtures did not differ significantly from those in the mon- oculture. Contrary to other reports, intercropping did not reduce aphid landing rates in this study; in fact, in soybeans, intercropping increased landing rates.

Innoculating soybeans with SMV. Photo by Gail Kampmeier.

However, although aphid landing rates at the level of the soybean canopy were greater in the mixtures, fewer of these soybean plants became infected with SMV than in the monoculture. Although not statistically significant, a similar trend toward a lower incidence of MDMV in corn in the mixtures than in the monoculture was observed, even though aphids tended to land more often in the mixtures than in the monoculture at the corn canopy. Among the mixtures, the incidence of both SMV and MDMV was inversely proportional to the number of nonhost plants of the virus. This inverse relationship was attributed to the increased probability that an aphid landing and probing on a nonhost plant of the virus would lose a substantial amount of its virus charge. Nonhosts of SMV and MDMV in the system played an extremely important role in reducing the spread of both viruses and probably overcame any disadvantage associated with increased aphid landing rates.

Even in extreme epidemic years, the slower accumulation of infected plants in a mixed cropping system can reduce the severity of the epidemic, lessening yield losses and maintaining seed quality, thus decreasing the chances of the virus being carried in seed to the next growing season or from one continent to another. Planting nonhosts of the virus in mixed cropping systems can therefore be recommended as a tactic to help minimize aphid-borne nonpersistently transmitted plant virus epidemics.

Gwendolyne Y. Fondufe, Michael E. Irwin, Harry Bottenberg, and Gail E. Kampmeier, Center for Economic Entomology


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