Potato Grower

36 Potato Grower | MAY 2014 diggin' in INSECT VECTORS / Ian MacRae, Dept. of Entomology, Univ. of Minnesota Making Sense of Phytoplasmas The cause of purple top wilt explained Phytoplasmas are probably best described as specialized bacteria. They lack a cell wall, have a filamentous shape and are pathogenic to plants having an obligate relationship with their hosts. They cannot be mechanically transmitted, requiring an insect vector for transmission to new hosts. Phytoplasmas have a wide host range and the diseases they cause are associated with their presence in the phloem (e.g. stunting or decline of the infected plant, unusual growth such as witches broom, phloem necrosis, dieback, and eventual death). The potential for yield impact is significant. There are a number of phytoplasma species associated with diseases in potatoes. Probably the two most notable are purple top—sometimes referred to as purple top wilt—and potato witches' broom. Purple top is caused by several strains of the aster yellows phytoplasma (AYp). Early symptoms of purple top resemble those of potato leaf roll virus: rolled leaves and yellow discoloration. Eventually these change to include unusual growth, including swollen nodes on stems, aerial tubers, stunting and elongation of the axillary buds (resulting in a really bushy looking plant). Symptoms usually show two to three weeks after the plant becomes infected, depending on the temperature (warmer temperatures accelerate the onset of symptoms). Purple top's impact on yield depends on when in the plant's life history it was infected. Plants infected prior to tuber development can suffer impairment of tuber development. If infected after tuber development, purple top can cause losses in processing potatoes through production of sugar in developed tubers. Aster yellows or purple top outbreaks in the North Central Plains is dependent on the annual population of its vector, the aster leafhopper. Because there is no rescue treatment for purple top, the best hope for managing the epidemiology of the disease is vector control. The aster leafhopper is sometimes referred to as the six-spotted leafhopper because of the six distinct dark markings on its head. It's a small, grayish-green insect (4 mm long), is ubiquitous in the North Central Plains and feeds on a wide variety of host plants. They differ from potato leafhoppers, also commonly found in potato fields, in that the potato leafhopper is smaller and a more vibrant shade of green as opposed to the grayish-green of the aster leafhopper. Aster leafhoppers have piercing/sucking mouthparts designed to feed on plant sap and when feeding, they inject saliva into the host to aid in making the food easier to ingest. The insects' feeding rarely causes yield loss itself, so no treatment thresholds have been developed for aster leafhoppers. Although aster leaf hoppers do overwinter in the North Central Plains, they don't often emerge until late June. Earlier season aster leafhoppers, present in fields from May onward, are migrants, blown up by low-level jet stream winds from the Gulf region. The deposition of insects from these low-level jets generally requires a storm front, as insects carried on these wind events are often rained out of them. Migrant individuals are considered more important to the epidemiology of purple top; they infect plants earlier, impacting tuber development, and present more visible symptomology. Leafhoppers acquire phytoplasmas from infected plants by feeding on sap. Reported acquisition periods (the time necessary for a vector to become potentially infective after acquiring the pathogen) for aster leafhoppers acquiring AYp varies from hours to days, but it is not an instantaneous occurrence. Aster leafhoppers can acquire AYp from a local host or from hosts in their overwintering area. So, migrant individuals may be arriving in the North Central Plains already infective. Once an aster leafhopper has ingested AYp from an infected plant, the phytoplasma moves across the mid-gut and enters the insect's blood. Insects have an open circulatory system, meaning blood circulates freely inside the body cavity. As AYp are circulating in the leafhopper's blood, they replicate and eventually make their way to the insect's salivary glands. This process (called the latency period) takes 10 to 21 days, meaning a leafhopper cannot transmit AYp for two to three weeks after acquiring the pathogen. Aster leafhoppers transmit AYp to uninfected plants in a relatively short period by injecting the phytoplasmas along with the saliva while feeding. The insect is actually affected by AYp as well. Female aster leafhoppers exposed to AYp live longer and have more eggs than unexposed leafhoppers, likely increasing the spread of the disease. It is possible to manage purple top with well-timed insecticide applications, although more than one application may be necessary. The period over which aster leafhoppers enter fields (two to three weeks) precludes relying on residual insecticide activity for control. In addition, trials in other crops indicate that, in the field, seed treatments may not be as effective as well-timed foliar applications. To assess the necessity of an insecticide application against aster leafhoppers, consider if there are symptoms of AYp infection in neighboring, susceptible crops. If not, there may be insufficient inoculum in the region to worry about transmission. If a rapidly developing population of aster leafhoppers is present (e.g. a large immigration event), nearby symptoms may not be visible yet. An alternative to insecticide, if possible, may be to harvest or top the crop prior to symptoms. If an insecticide treatment is applied, keep in mind that additional insecticide applications may be necessary; leaf hoppers recolonize fields within two weeks and insecticides are unlikely have long enough residual activity. Synthetic pyrethroids have been shown to flare aphid populations, so if used, developing aphid populations should be scouted. Finally, if a field has already been treated with a neonicotinoid insecticide for Colorado potato beetle control, use a different mode of action against aster leafhoppers. This will minimize the contribution of potential development of resistant beetle populations. PG

• Cover