Researchers find management practices can reduce zebra chip occurrence
Researchers are making important advancements in their understanding of zebra chip disease of potatoes, which will help producers as they begin their planting season, starting in South Texas.
Zebra chip incidence may be related to both environmental factors and management practices, according to studies by Dr. Don Henne, assistant research scientist, and Dr. Fekede Workneh, research scientist, both with Texas AgriLife Research in Amarillo, as well as other researchers working on the control of the disease.
Henne and Workneh both work with Dr. Charlie Rush, an AgriLife Research plant pathologist who is leading a team of 20 researchers and Extension specialists in a federally funded specialty crop research initiative titled, "Management of Zebra Chip to Enhance Profitability and Sustainability of U.S. Potato Production."
The work of the two researchers, as well as that of many others, was presented during the Citrus Huanglongbing and Potato Zebra Chip Conference just two weeks before the new season begins in the McAllen area.
Zebra chip is caused by the Liberibacter bacteria, which is spread by a tiny insect called a potato psyllid. Not all psyllids carry the bacteria. However, the psyllids also are identified in spreading the citrus huanglongbing or citrus greening disease, and that is why both were discussed at the conference.
Henne said one of their studies focuses on monitoring the spread of the zebra chip disease, both over time and space. In two performance studies conducted in potato fields near Olton and Dalhart, psyllids were collected on sticky traps from June 4 to July 7, as well as in 100 manual sweeps in the field with insect nets weekly at six locations per field.
Within two weeks, the psyllid numbers had maxed out, he said, but the relationship in insect numbers and incidences of zebra chip were not as expected.
"There was no relation between the numbers of psyllids and the disease incidence--not that we found," Henne said. He added it could be because the number of psyllids present was low and the infective ones were missed.
What he did find was that nearly 50 percent of the captured psyllids had just emerged in the area where they were sampled, indicating they did not fly to the fields. This suggests within-field population growth, and that initial psyllid arrival was well before the collection dates, probably before bloom period.
Henne said the initial arrival of psyllids at Olton could be as early as mid-May, which agrees with information gathered in other surveys. He also found that while 94 psyllids were collected, only six tested positive for Liberibacter.
This low level of Liberibacter-infective individuals within psyllid populations appears to be a real phenomenon, he said.
The hyposthesis is that psyllids are actively moving among plants, feeding on them and thereby infecting them, Henne said, adding incidence of zebra chip was higher in the south, west and center part of the fields.
Henne said they also found that Liberibacter levels within psyllids appear to be sensitive to high temperatures, but they will conduct additional testing to verify this.
Another study by Workneh and Henne evaluated the edge factor in occurrence of zebra chip. Verification of this could help producers direct more management practices towards the edges of fields and perhaps reduce their chemical costs by not having to treat the entire field as often.
The edge-effect studies were conducted in three fields, with paired plots at multiple locations around each field, Workneh said.
"We saw that in most of the cases, the edge had significantly higher ZC incidences, so we think there really is an edge effect," he said. "There are pockets where the infield has greater incidence of psyllids and disease but, in general, the edge does.
"One of the many possible explanations for the aggregation of infected plants is that we think maybe the first to land is a female and she attracts the males all around," Workneh said. "Every week when we looked at it, there were new fresh symptoms that were not there the week before."
He said after three weeks of observation, they began seeing a decline in the appearance of new zebra chip infections. Of all the plants that were marked each week as diseased, about 89 percent displayed the typical symptoms of streaking and necrotic flecks on the tubers.
Workneh said they also took the potatoes and stored them for about two months. Those affected by zebra chip would shrink at a faster rate, showing excessive shrinking after only a few weeks.
Dr. John Goolsby, a research entomologist with the U.S. Department of Agriculture-Agricultural Research Service in Weslaco, conducts a year-round regional monitoring program of potato psyllid populations in the Rio Grande Valley, Pearsall, Olton, Dalhart, Garden City, Kan. and several locations in Nebraska.
Potato psyllids feed on potatoes and transmit the bacterial pathogen that causes zebra chip. While chemicals such as Oberon, Fulfill, Sponto and Rinon have been added to the integrated pest management tools producers can use, Goolsby is trying to determine how the populations vary between growing areas and if season-long control of potato psyllids reduces the incidence of zebra chip.
From Jan. 5 to Oct. 15, more than 60 growers, researchers and processors received his weekly report from 3,000 traps and combined with leaf counts from 10,000 lower-canopy potato leaves to determine the nymph population.
Goolsby reported higher numbers of psyllids were found in every single growing area in 2009; the difference being how well the adults were managed once detected.
"We saw some good control by insecticides in some areas," he said. "But to me, the nymphs tell the story of how well your pest management program is working. Ground-based applications are more effective than aerial, and we also have learned that we have an edge effect. Most of our sampling has been from the edge of fields, so we are sampling the worst case scenarios."
Goolsby said even if a lot of psyllids are present, there are good insecticides out there for early control. Some of his other findings include:
--Integrated pest management programs are very effective at controlling psyllids.
--Percentage of psyllids with the Liberibacter pathogen varied between areas, with McAllen having the highest rate, followed by Pearsall, Olton, Dalhart, Garden City and Nebraska.
--A larger sample size of adults is needed early in the season for pathogen assay.
--The percent Liberibacter may vary between years and regions.