Lettuce Continued from Page 7
While the effort to identify the most important host plants continues, other researchers are working to discover ge- netic sources that could produce lettuce varieties with resistance to the disease. “We are looking at INSV resistance; we looked at 36 varieties and 78 breed- ing lines,” said Kelly Richardson, a U.S. Department of Agriculture research ge- neticist based in Salinas. “We planted on Aug. 19 to catch the maximum thrips and disease pressure. Seventy percent of
the plants in our experiment were show- ing symptoms.” Although the USDA greenhouse tests and field surveys have shown some lettuce varieties to be better able than others to withstand INSV, the resistance tends to be inconsistent. “Red varieties show less disease un- der low to moderate pressure, but that difference largely disappears as the pressure reaches a peak,” Richardson said. “Closed head varieties also show
less disease under low pressure but that difference, too, disappears under high pressure.” There are a few promising variet- ies that have suffered disease dam- age less frequently than others in the USDA screens. “The leaf varietiesCavalryandEruption and the romaine variety Rubens Red are still showing low disease incidences un- derhighpressure,”Richardsonsaid. “They show the lowest severity and the lowest disease incidence.” While USDA researchers continue to look for greater genetic resistance, they have released some germplasm they think could help the seed companies. “We have some material we are ready to release,” Richardson said. “We were able to find material that looked good under both growth room and green- house conditions. In late season plant- ing, look for something with closed ar- chitecture, which seem to have a lower incidence of INSV.” Researchers can also benefit from their store of knowledge about anoth- er viral disease with which they are more familiar. “INSV is related to tomato bushy s t unt v i r us , abou t wh i ch a l o t i s known,” said Richard Michelmore, di- rector of the University of California, Davis genome center. “We can build on that knowledge.” Tomato bushy stunt virus, like INSV, is vectored by thrips and causes similar symptoms on infected lettuce plants. Researchers are also looking to see if they can apply some of the most ad- vanced microbiological tools to man- agement of western flower thrips and the disease it carries. “We are working on RNA approaches to both thrips and INSV,” Hasegawa said. ( Bo b J o hn s o n i s a r e p o r t e r i n Sacramento. He may be contacted at bjohn11135@gmail.com.)
The top 10 Salinas Valley INSV weed hosts are little mallow, shortpod mus- tard, annual sowthist le, net t leleaf goosefoot, mare’s tail, field bindweed, shepherd’s purse, common purslane, hairy fleabane and burning nettle. During the annual lettuce-free pe- riod the most important virus hosts look to be, according to the sam- pling, chickweed, hairy fleabane and annual sowthistle.
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8 Ag Alert July 21, 2021
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