Bloom and midsummer are the two busy times. For almond brown rot blossom blight, UC IPM says to treat at pink bud, when 5 to 10 percent of the flowers are open, and again at full bloom. In most years with little rain, one spray at full bloom is enough. A long, wet bloom may need a second or even a third spray near full bloom. Wet weather at bloom is also when an orchard floor can be too soft for a sprayer. In walnuts, husk fly adults come out from early June to early September, with the peak from July to mid-August. UC IPM prefers bait sprays that kill the adults before they lay eggs. For prune rust, it says to check orchards every week from May 1 to July 15 in the Sacramento Valley, and to treat at the first sign. Your PCA sets each date.
Sources: UC IPM, almond brown rot blossom blight; UC IPM, walnut husk fly; UC IPM, prune rust
It depends on the pest, the product and the label. For some targets, partial coverage is enough. The UC IPM walnut husk fly guideline says that for low to moderate numbers, coverage is not critical, and partial coverage or low-volume bait sprays can work. For high numbers in an orchard with past damage, it calls for high-volume, full-coverage sprays, or more than one. Some labels decide for you. In the UC IPM almond brown rot table, one fungicide is listed for 400 gallons of water per acre, far more than an aircraft carries. Many growers use both methods: the plane when the window is short or the ground is wet, and the airblast sprayer when the floor is firm. Your PCA makes the call for each block and each spray, so ask before the season starts.
Sources: UC IPM, walnut husk fly; UC IPM, almond brown rot blossom blight
Orchards cover a lot of ground here. Sutter County's 2025 crop report lists 36,700 bearing acres of walnuts, its second crop by value at $115.6 million. The same report lists 12,200 bearing acres of prunes and 7,840 of clingstone peaches. In Colusa County's 2024 report, almonds are the top commodity, at $279.0 million from 63,017 acres. Walnuts and prunes are among the main crops in Butte, Glenn and Yuba counties too. Big acreage and short windows go together. When rain moves through at bloom, many blocks need a spray in the same few days. Send your blocks and your PCA's timing before the season starts, so your orchards are on the schedule. For each block, a field address, an APN, or an FSA farm and tract number is enough to start. We map the block and check the hazards around it.
Sources: Sutter County 2025 crop report; Colusa County 2024 crop report
