Walnut blight can affect all varieties, but it’s most serious on early blooming cultivars. Walnut orchards with high populations of over wintering bacteria may be seriously damaged during years favorable to the development and spread of the pathogen.
All green tissue is susceptible to walnut blight infection, including buds, flowers and leaves, but nut infections are the most serious, usually resulting in economic damage. Early in the spring, infected walnuts develop a dark sunken lesion at the blossom end (end blight) killing the developing kernel.
As walnuts mature, blight lesions can develop elsewhere on the husk surface (side blight). Husk lesions begin as small water-soaked spots that later darken, enlarge, sink and often crack. Infections that do not invade the kernel may increase the possibility of secondary insect or disease damage.
The walnut blight pathogen, Xaj, survive the winter in the outer bud scales of dormant buds. As the new shoot elongates after bud break, Xaj are water transported from the outer bud scales to green tissue, flowers and developing nuts. Disease control requires applying spray materials to protect green tissue. If favorable conditions exist, secondary inoculum can lead to additional infection and inoculum buildup.
If dormant bud populations are low and sprays protect the developing nuts and newly formed buds, infection does not occur. A successful walnut blight control program focuses on protecting developing shoots and flowers and decreasing Xaj bacteria over-wintering in dormant buds.
The probability of infection depends upon how much pathogen exists on individual buds and environmental conditions favoring bacterial spread and infection. First blight sprays are timed to coincide with early shoot emergence. This places a protective layer of bactericide on emerging green tissue.
If bacteria are splashed from the outer bud scales to developing shoots and flowers, the bactericide barrier prevents infection. Since all shoots do not emerge at the same time, a good compromise is to apply the first spray application when 40% of the shoots on the tree are at the “prayer stage” when unfolded leaves resemble hands held in prayer.
A second spray is applied about 7 to 10 days later to protect the remaining opening buds. Additional sprays may be necessary depending upon the initial inoculum, disease history, weather conditions and variety.
If you want to be more conservative and/or initial inoculum (disease pressure) is high, apply the first application at 20 % prayer stage or sooner, and a second application 7 days later. Either way, the early sprays are critical for success. Additional sprays are advised with high inoculum levels and wet weather particularly for sensitive varieties.
Reducing serious blight damage has often taken 2 years with an aggressive spray program. Good coverage is essential for walnut blight control. Spray materials have to cover walnut tissue with an adequate amount of spray material to protect the green tissue. It is possible that half spray programs favor increasing populations of Xaj and the half spray-sub lethal exposure approach is an excellent way to select for resistance to the only effective spray program we have for walnut blight.
Copper products tank mixed with Mancozeb is the best available spray for walnut blight management. Resistance to copper alone is very common in California walnut growing areas and is thought to be responsible for poor walnut blight control in the early 90’s prior to EBDC tank mixes.
Since 2012, Dr. Jim Adaskaveg at UC Riverside has been investigating Xaj sensitivity to copper plus Mancozeb tank mixes. Jim found a 2X to 6X shift in Xaj sensitivity to copper Mancozeb mixtures. Although a 2X to 6X shift is rather small, it indicates the potential for adaption of the pathogen that could lead to further decreased sensitivity. This research does suggest the crucial need for alternative materials for walnut blight management.
Our experience still suggests that any good quality copper compounds will provide good control when tank mixed with Mancozeb. Under heavy blight pressure, using maximum rates allowed by the label is suggested. Pesticide regulations change. If in doubt regarding pesticide labels, check with your local Agriculture Commissioner.
What Can Go Wrong
Over several years we have intensively monitored the amount of bacterial inoculum in dormant buds, grower spray programs and the percent of walnut blight damage for 30 orchards in Butte and Tehama counties. Varieties include Chandler, Howard, Hartley, Tulare, Vina and Ashley. We observed the following possibilities when evaluating walnut blight control failures.
- First spray timing too late.
- Walnut blight bacterial population increases in dormant buds resulting in high initial disease pressure.
- Material rates too low.
- Poor spray coverage by air or ground.
- Using a weak bactericide (non-coppper/Mancozeb mixture) in high blight potential orchards.
- Not tank mixing copper compounds with a Mancozeb formulation.
- Dense tree canopies.
Mancozeb Products Available for California Walnuts
For several years, manganese ethylenebisdithiocarbamate (EBDC) formulations have provided the backbone for walnut blight control in California. The California walnut industry currently has supplemental labels and special local need labels for Mancozeb use in California. Click on the following to enlarge the view:
Until the California Department of Pesticide Regulation (CDPR) approves amended labels, applicators must still use the old supplemental labels and Special Local Need (SLN) if applicable. All supplemental and SLN labeling must be in possession at the use site and are not delivered with the original product.
These labels are available via the internet at sites like Agrian or CDMS. Your County Ag Commissioner or pesticide dealer can provide a copy as well. Supplemental and special local need labels often change, so double check with your local Agricultural Commissioner for updates regarding these products and other formulations if they become available.