Walnut orchards at the Nickels Soil Lab near Arbuckle, Calif. are saturated with Botryosphaeria, (BOT) and Phomopsis blight and canker, says University of California plant pathologist Themis Michailides. This makes a difference in management practices.
In heavily infected orchards where more than 50 percent of the trees show signs of the disease, prunings can be left in the orchard and a multiple spray fungicide program used for control. In moderately infected orchards, prunings should be removed and burned, plus some fungicide spraying is recommended.
With very low infection rates, prunings should also be removed and burned. One mid-June to early July spray is advised.
In young orchards with no BOT infection, prunings can be chipped and left in the orchard. In light to moderately-infected orchards, Michailides says chipping pruned wood and removal is the best option to reduce infection. He advises pruning or hedging more lightly in infected orchards before moving equipment into heavily infected sites.
Management integrates cultural and chemical control practices. The removal of infected wood, disinfecting pruning equipment between orchards, and avoiding irrigation that wets the tree canopy are good cultural practices. Applying effective fungicides at the right time is the second control component.
Michailides is based at the UC Davis Kearney Research and Extension Center in Parlier. He says rainfall favors BOT and Phomopsis canker and blight of walnut when temperatures rise above 50 degrees Fahrenheit. Both pathogens produce abundant water-splashed and wind-borne spores and both types require water to trigger infection.
Infection requires susceptible tissue, active spores, and the right environmental conditions - at least a quarter inch of rain or irrigation water hitting susceptible tissue and temperatures at 50 degrees or higher.
Infections reduce walnut yields by killing small fruitwood and large branches, and directly infecting the nuts. Visual signs are blighted shoots with leaves attached, plus nuts with the entire hull blackened and still on the tree.
Michailides says calendar sprays are effective to reduce canker and blight. The sprays can start at bloom and end by late July. If only one fungicide application is made, the second half of June to early July has had a more positive effect when tested in low and high infection rate orchards.
Growers seeking to reduce the number of fungicide applications annually can use the leaf wetness model where applications are only made when environmental conditions are conducive to disease presence.
Michailides says timing sprays - based on rain and temperature - delivered the best control during the 2016 trials.
Using a leaf wetness model based on rainfall amount and temperatures determines the risk zones for BOT and the need for a fungicide application. As the duration of leaf wetness accumulates and temperatures rise, the risk of infection increases.
Using the leaf wetness model to trigger fungicide applications only when environmental conditions are conducive to BOT infection may save one to two fungicide applications per season, Michailides says. The leaf wetness period, in hours, begins when rain starts and ends, plus a half hour if skies are clear or one hour if overcast.
“This may be the most important finding if we continue to have wet springs,” Michailides said.
In trials using the leaf wetness model, three fungicide applications were made during 2015 and 2016. Fungicide applied 2-3 days prior to a rain event had an estimated two-week residual effect.
There are several fungicides which have reduced blighted spurs compared to the control. Research is ongoing to determine the most effective fungicides and the best timing of treatments for BOT management.