New dis­ease hits Vi­dalia onion yields

The Standard Journal - - Farm & Garden -

Ge­or­gia is the only state that pro­duces sweet Vi­dalia onions. It’s also the only state where onion farm­ers are tack­ling a new dis­ease - yel­low bud.

Yel­low bud (Al­lium Cepa) turns onion leaves yel­low, sim­i­lar to a ripe banana. The plant ei­ther dies or loses the yel­low color in the leaves and greens back up. While the plant looks healthy, the dam­age has al­ready been done.

Onion farm­ers could get to har­vest time, never know they had the dis­ease and face sig­nif­i­cant yield losses, said Ron­ald Gi­taitis, a Univer­sity of Ge­or­gia plant pathol­o­gist who is study­ing the dis­ease.

“Over the last sev­eral years, it has spread wider and wider through­out the onion grow­ing area,” Gi­taitis said. “It started in a small area in Tat­tnall County and now it’s spread through­out vir­tu­ally the en­tire re­gion.”

Yel­low bud is a cold weather pathogen that is at its worst in Jan­uary and Fe­bru­ary. Gi­taitis be­lieves the state’s cooler grow­ing sea­sons are the rea­son the dis­ease has been con­fined to Ge­or­gia.

Ge­or­gia onions are grown in the win­ter, which pro­vides the per­fect tem­per­a­tures and con­di­tions for yel­low bud to thrive.

“Most onions are grown through­out the summer in north­ern ar­eas when it is warmer,” Gi­taitis said. “How­ever, a few other ar­eas also grow sweet onions dur­ing the win­ter, like New Mex­ico and Texas, but I think our win­ter con­di­tions are a lit­tle bit dif­fer­ent than theirs, which is why we’re see­ing it. It’s also pos­si­ble that the bac­terium has not yet found its way to those ar­eas.”

The pathogen that causes yel­low bud is es­pe­cially danger­ous be­cause it spreads through the air. If it rains or if farm­ers are ir­ri­gat­ing their onion crop, the wa­ter will knock the pathogen out of the air and onto the crop, said Gi­taitis.

The bac­terium then lives on the leaf sur­face and sur­vives on nu­tri­ents leeched out of the leaf. When it grows, the bac­terium pro­duces a toxin that causes chloro­phyll to turn yel­low.

If tem­per­a­tures are cooler and the bac­terium pop­u­la­tions de­cline, the toxin lev­els de­crease and the leaves’ green color re­turns. “There is a re­mis­sion of symp­toms, but onion pro­duc­tion is still de­creased be­cause of a loss in fo­liage,” he said.

Study­ing the dis­ease on the UGA cam­pus in Tifton, Gi­taitis has found the best method of con­trol is cop­per bac­te­ri­cide.

“The-go-to rec­om­men­da­tion for bac­te­rial dis­eases is to spray with a fixed cop­per bac­te­ri­cide, but it may be par­tic­u­larly dif­fi­cult to con­trol this bac­terium in that man­ner as the tim­ing of the spray is su­per crit­i­cal,” Gi­taitis said. “The spray has to be ap­plied in ad­vance of bac­te­rial de­po­si­tion. Once es­tab­lished on the leaf sur­face, this bac­terium can col­o­nize the hol­low cen­ter of onion leaves and es­cape the bac­te­ri­ci­dal cop­per residue on the leaf’s ex­te­rior from that point for­ward.”

Gi­tatis also sug­gests onion grow­ers pull in­fected plants when the dis­ease is spot­ted in seedbeds.

“I re­al­ize avoid­ance is eas­ier said than done, es­pe­cially if the dis­ease is wide­spread, but un­til re­search [finds] al­ter­na­tive con­trol mea­sures, we are pretty much lim­ited to rec­om­mend­ing avoid­ance and spray­ing with cop­per,” he said.

Ac­cord­ing to the UGA Cen­ter for Agribusi­ness and Eco­nomic Devel­op­ment, Ge­or­gia’s onion crop gen­er­ated $163 mil­lion in farm gate value with Toombs and Tat­tnall Coun­ties lead­ing the state in acreage and farm gate value in 2012.

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