Walnut blight (Xanthomonas campestris pv. juglandis) control investigations in Northern California, USA

Walnut Blight disease caused by the bacteria Xanthomonas campestris pathovar juglandis is a very destructive disease for walnut farmers in Northern California. Infections destroy the kernel causing significant crop loss, particularly for early leafing varieties in high rainfall areas. Copper compounds are traditionally used for walnut blight disease control. Tank mixes of copper plus Manex (R) significantly improve disease control compared to copper alone. Copper at 4.49 kg active ingredient mixed with 4.24 liters of Manex (R) per sprayed hectare is the best currently available spray for Northern California. Streptomycin is an effective treatment; however, nut drop is a problem if Streptomycin is applied at the wrong time. Zinc formulations have activity against walnut blight but can be phytotoxic. Lime is added to Zinc Sulfate to decrease the "phyto" problem, however, Zinc/Lime is difficult to tank mix and sprayer clogging is a problem. Iron Chloride has worked well when mixed with copper hydroxide, magnesium sulfate and the adjuvant CS7. Neither zinc or iron have performed as well as the copper/Manex (R) mix. New products such as Actigard (R), B694, DTEA, DBNPA and PHMB show promise as blight control materials if the "stickability" to walnut tissue can be improved. In Northern California, first sprays are applied when significant susceptible tissue is present, usually between pre-bloom and 1% bloom. Weekly applications are typical depending upon rainfall and complete spray coverage is essential.