Winter-growing weeds such as burr clover, malva and sowthistle are known to rob pepper plants of moisture and nutrients. They are also hosts for viruses that attack the crop, so winter weed control to break the viruses' life-cycles might be worth considering.
Monterey County farm advisor Richard Smith says it's not a documented fact, but he suspects the weeds supply much of the fresh tissue viruses need to survive until they are transmitted to a new pepper crop.
Other crop and ornamentals are also hosts of tomato spotted wilt and other viruses, he told a recent gathering of pepper growers and PCAs at Gilroy.
He said if he were a grower, he would give attention to controlling winter weed hosts of viruses, even without proof it works, to do everything possible to manage virus problems.
“Last year was so devastating with viruses, we have to do as much as we can to reduce the amount of inoculum out there.”
The problem may be bigger than anyone knows, he added, and perhaps even fava bean cover crops are symptomless carriers of viruses.
General weed control is essential in peppers, since they are poor competitors and must face multiple waves of weed emergence during their long growing season.
Growers should remember, Smith said, that common names of viruses — for example, tomato spotted wilt or cucumber mosaic — give a false impression that they occur only on those crops, when actually they go to long lists of host plants.
“During a devastating outbreak of cucumber mosaic in crops during the 1980s it was also observed that another virus host, periwinkle, was abundant in riparian areas, so there may be a connection,” he said.
While several non-chemical weed controls may be employed, peppers fortunately have a number of herbicides registered for use preplant through postemergence. One recent addition was a 24(c) registration for Syngenta's Dual Magnum, which Smith said gives good control of black nightshade, a host of tomato spotted wilt virus.
In his 2004 trials on weed control for peppers, Smith evaluated Dow AgroScience's new formulation of the pre-emergence Goal 4F, or Goal Tender, and Goal 2XL and also experimented with application of Dual Magnum after transplanting. He said the results of the trials were very encouraging for a new system of longer weed control.
The label for Goal 2XL pre-emergence on peppers calls for use 30 days before transplanting but requires working of the beds before transplanting to avoid crop-damaging lift-off of the material.
“The problem with this,” Smith explained, “is that when the beds are worked the layer of protection is destroyed. We wanted to see if we could apply Goal Tender, the new formulation potentially having less lift-off, and transplant into the layer without working the bed, to get 30 days of weed protection after transplanting.”
Then Smith applied Dual Magnum at layby with the hope of another 30 days of protection. “The idea was to develop a system so we could get deeper into the season with weed control.”
The results showed either Goal Tender or Goal 2XL at the 0.5 pound active ingredient per acre rate applied 15 days prior to transplanting gave excellent weed control for the first 30 days following transplanting.
“Goal Tender,” Smith reported, “appeared to be safer to peppers, but there was no difference in the yield of peppers treated with Goal 2XL or Goal Tender.”
The Goal and Dual Magnum combination, he said, “looked promising as an extended season weed control treatment for more than 30 days following pepper transplanting.
“This is a really interesting idea and now we have to work with the manufacturers to make some label changes, although these could take a number of years.”
Smith also investigated the new Valent USA flumioxazin materials, Chateau and BroadStar, in the trials. The liquid Chateau at all rates, he said, gave excellent weed control but was too toxic on peppers as a preplant application.
However, the granular BroadStar gave excellent weed control and acceptable safety when applied over-the-top, post transplant.
A recent additional tool to combat malva and other weeds is the use of Goal 2XL under plastic tarps to protect the crop from lift-off damage. Escapes of some weeds, particularly malva, may occur with typical metam-sodium fumigation.
Tarped Goal applications have been used in southeastern U.S. on transplanted peppers, strawberries and tomatoes, and the method was only recently approved for use in California, Smith said.
Green peach aphid
Trials in California in strawberries last year revealed that both Goal 2XL and Goal Tender, when tarped, caused significantly less damage to plants than when they were not tarped.
In a talk on insect management on the Central Coast peppers, Bill Chaney, Monterey County farm advisor, said green peach aphid is an economic pest of peppers along the Central Coast because it vectors several viruses.
“This aphid hadn't been a very big problem for us for several years, but last year was the ‘year of the green peach aphid.’ Growers in Santa Maria still don't believe it was the aphid on their cole crops, but it was,” he said.
Like “a dirty needle” among drug addicts, the aphid inserts its stylet into an infected plant, picks up virus particles, and spreads them to a clean plant. If it doesn't like the taste of a plant, it moves on and on, distributing the virus, as it probes.
Moreover, the green peach species has a wide host range and operates more widely than, say, a lettuce aphid that is confined to a group of plants.
Beyond vectoring viruses, the aphid mars produce with honeydew excretions that bring on sooty molds.
Although numerous insecticides are available for aphid control, they are not successful in managing virus transmission, Chaney pointed out.
“An aphid has to be on a treated plant for a few minutes to pick up enough insecticide to kill it, but during that time it transmits the virus.”
However, controlling aphid populations in a field may be helpful in slowing the spread of viruses in that field, even you can't keep them from coming in, he added.
Thrips, primarily western flower, vector tomato spotted wilt virus to peppers, and management alternatives are insecticides and control of weed hosts.
“The problem with trying to manage thrips with insecticides is the infected adults that migrate into a field, providing a constant re-infection, even when you control them in a single field,” Chaney said.