Ag Alert May 26, 2021

Cal Poly plot tests avocado trees for root rot resistance

soil, because this is a root rot trial,”he said. “Then we deep ripped the 12- to 14-inch hardpanandsetupbermsparallelwiththe slope to promote drainage. After the trees were removed, thedeep rippingandberm building cost $4,500 to $5,000 an acre.” Avocado growers with groves on hilly groundshouldbeable toachieveadequate drainagewithout investing in berms. “On f lat ground, you can af ford this equipment,” said Ben Faber, UC Cooperative Extension farm advisor. “When you’re on a 15-to-20% slope, you shouldhavegooddrainage.At a15- to-20% slope, theadvantageof bermsdisappears.” With the trees planted in berms to promote drainage, the next challenge was to prevent storms from washing away the berms. “We planted a barley, wheat and oats cover crop mix to prevent erosion of the berms,” Rosecrans said. Garner saidthemixof cereal cover crops has had an added benefit. “The cover crop is doing anice job sup- pressing the weeds between the rows,” she said. Another trial at Cal Poly is looking at the benefits different cover crop mixes can bring to avocado orchards. “Withavocados, youcanusecover crops to improve soil health,” said Charlotte Decock, Cal Poly assistant professor of soil science. “It can play a role in newer orchards where the canopy hasn’t closed and kept the sun out.” For avocados, she said, Faber had rec- ommended “using something that’s not going to grow too tall.” Decock is overseeing an orchard cover crop trial using different mixes at two lo- cations inthegreaterSanLuisObispoarea. “In the Edna Valley, we planted a fal- low treatment, a triticale cover crop and a triticale withmycorrhizae treatment,” she said. “AtCal Poly,weplanteda fallowtreat- ment, triticale, anda10%triticaleand90% legume cover cropmix.” Some cover crops can be managed so they come back every year without taking the timeandmoneyneeded toplant them again, she said. “You can let your cover crop reseed it- self, soyoudon’thave toreseedeveryyear,” Decock advised. ( 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.)

Avocado trees can be sus- ceptible to Phytophthora

root rot. Potential solutions include resistant root- stocks being test- ed by California researchers.

ByBob Johnson A small block of avocados planted on the Cal Poly San Luis Obispo campus has given campus researchers anopportunity to study root rotmanagement strategies. The3acresof struggling treeswerebull- dozed, chipped andused asmulchbefore an avocado rootstock trial was planted in berms, as part of a statewide effort to find answers to Phytophthora, a root rot that plaguesmany avocado operations. “We had a block with a hardpan that was suffocating the trees,” said Johnny Rosecrans, senior farm technician at Cal Poly. “We replanted it as part of the University of California, Riverside, Phytophthora root rot resistance trial.” Rosecrans made his remarks as re- searchers discussed efforts to man- age Phytophthora root rot during a Zooming to Healthier Trees and Soils webinar, co-sponsored by the California Avocado Society, the California Avocado Commi ss ion and UC Cooperat ive Extension. Phytophthora, themost important dis- ease in avocados, thrives in areas with ex- cess soil moisture and poor drainage. The disease reduces feeder roots to the point that the trees produce pale, yellowish leaves and cannot set a good crop. ThePhytophthorapathogens still in the ground from the previous diseased trees made the 3-acre block at Cal Poly ideal for testing rootstocks that are supposed to have resistance to the disease. “There’s no bet ter place to tes t Phytophthora-resistant rootstocks than in anarea known tohavePhytophthora,” said LaurenGarner, a horticulture professor at CalPoly.“Thisisthenorthernmostsiteofthe UCRiverside-basedtrial,andit includesthe PP35, PP40andPP45 rootstocks.” This Southern California trial is look- ing at new rootstocks that could help avocado growers contend with several potentially damaging diseases, includ- ing Phytophthora. “UCRiversidehasseveralnewrootstocks they hope to release soon,” Garner said. “Wehaveplanted threeof those rootstocks andDusa, which is an industry standard.” In addition to the Phytophthora study, the rootstocks are alsobeing evaluated for tipburnfromsalinity, heatdamage, bloom and flushing.

“Wehavealreadybeenable toseediffer- ences,”Garner said. “Oneof the rootstocks resulted in trees that are taller than those grown on Dusa, and there is also a differ- ence in Perseamite damage.” Researchers said the apparent vigor of the trees on some of the new rootstocks couldprove invaluable ifCaliforniaavoca- dogrowers have to contendwithanexotic fungal disease that can kill trees within weeks after infecting the vascular tissue. “If the trees are healthier, these root- stocksmight helpwith laurel wilt disease, should the beetle that carries it arrive in California,” Garner said. The redbay ambrosia beetle, whichhas not yet reached California, carries the laurel wilt fungus to avocado trees, and the UC Riverside researchers are hoping to find resistance to this disease aswell as to Phytophthora. Even the scientists at Cal Poly said they didnot know the blockwas suffering from Phytophthora until the old trees were re- moved and the soil and roots analyzed. “ B e f o r e w e k n e w a b o u t t h e Phytophthora, when the trees started to wilt, we tried applyingmore water, which didn’t help,” Garner said. The next trees were planted on berms to promote drainage, which is a strategy that Rosecrans said only pencils out in some situations. “Wehadtoremove theexisting treesand grind themup before land prep could be- gin; wewanted the pathogen to stay in the

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