Cause Xanthomonas arboricola pv. juglandis (formerly Xanthomonas campestris pv. juglandis), a bacterium that overwinters in infected buds and catkins. Buds with the highest bacterial populations are the ones most likely to develop blight. During early spring growth, bacteria spread along developing shoots and nuts. There seems to be very little secondary spread to other shoots and trees by raindrip. This results in local infection centers within a tree or orchard. Frequent, prolonged rain, just before and during bloom and for about 2 weeks after, result in severe blight outbreaks within these local infection centers. This is when nuts are most susceptible. Buds for next year's shoots are colonized very soon after their formation in the spring. Although the 'Howe' cultivar is resistant to blight in Oregon, most commercial cultivars are susceptible. 'Ivanhoe' is extremely susceptible.
Symptoms On leaves, infection appears first as reddish-brown spots, on the stems as black, slightly depressed spots often girdling the shoots. Young, infected leaf and catkin buds turn dark brown or black and soon die. The disease is serious on nuts, where it causes black slimy spots of varying sizes. The organism penetrates the husk, the shell, and occasionally the edible meat. Late-season infection produces black rings on husks. Nuts may fall prematurely.
Cultural control
- Prune out diseased twigs and branches, if practical.
- Use low-angle or microsprinkler heads, that do not let water wet trees. Drip irrigation can also be used.
Chemical control Spray at early prebloom (when catkins begin to enlarge), late prebloom (when shoots begin to expand), and early postbloom. In California, initial treatments are recommended when the terminal buds break. Resistance to copper products (Group M1 fungicide) has been seen widely in California and may be a problem in the Pacific Northwest. However, copper resistant bacteria do not seem to cause as much disease as ones that are copper sensitive. Addition of mancozeb (Group M3 fungicide) will boost the copper-ion concentration and may result in improved disease control. Bacteria, however, will become resistant to the higher-copper concentrations. Most spray adjuvants have not improved blight control; however, silicone-based adjuvants show promise.
- Badge X2 at 3.5 to 11 lb/A. Group M1 fungicide. 48-hr reentry. O
- Bonide Liquid Copper Fungicide at 0.5 to 2 fl oz/gal. H
- Bordeaux 4-2-100. Group M1 fungicide. O
- Champ WG at 4 to 8 lb/A. Group M1 fungicide. 48-hr reentry.
- C-O-C-S WDG at 4 to 7.7 lb/A. Group M1 fungicide. 48-hr reentry.
- CS 2005 at 32 to 51 oz/A. Group M1 fungicide. 48-hr reentry.
- Cueva at 0.5 to 2 gal/100 gal water/A. Group M1 fungicide. 4-hr reentry. O
- Cuprofix Ultra 40 Disperss at 5 to 10 lb/A. Group M1 fungicide. 48-hr reentry.
- Dodine FL at 3 pints/A is legal to use as a mixing partner with copper-based products. Do not use within 7 days of harvest. Group U12 fungicide. 48-hr reentry.
- Grotto at 203 gal/A Group M1 fungicide. 4-hr reentry. O
- Instill at 30 to 50 fl oz/100 gal water. Group M1 fungicide. 48-hr reentry.
- Kasumin 2L at 64 fl oz/A in at least 100 gal water/A plus another bactericide from a different FRAC group. Do not use alternate-row applications or within 100 days of harvest. Do not use in orchards fertilized with animal manure. Moderate efficacy alone, good when tank mixed. Not for nursery production. Group 24 fungicide. 12-hr reentry.
- Kocide 50 DF at 5 to 8 lb/A. Group M1 fungicide. 48-hr reentry.
- Mancozeb-based products can be used as mixing partners with copper-based products. Do not use within 75 days of harvest. Group M3 fungicides. 24-hr reentry.
- Dithane F45 at 1.8 quart/A plus a fixed-copper product.
- Manzate Pro-Stick at 2.4 lb/A plus a fixed-copper product.
- Penncozeb 75 DF at 2.4 lb/A plus a fixed-copper product.
- ManKocide at 6 to 10 lb/A. Do not apply within 75 days of harvest. Group M1 + M3 fungicide. 48-hr reentry.
- MasterCop at 3 to 6 pints/A. Group M1 fungicide. 48-hr reentry.
- Monterey Liqui-Cop at 3 to 4 Tbsp/gal water. H
- Nordox 75 WG at 5 to 8 lb/A. Group M1 fungicide. 12-hr reentry. O
- Nu-Cop 50 DF at 4 to 8 lb/A. Group M1 fungicide. 48-hr reentry. O
- Previsto at 2 to 4 quart/A. Group M1 fungicide. 48-hr reentry. O
- Regalia at 1 to 4 quarts/A. Does not benefit from the addition of an adjuvant. Use on 7-day intervals and cover trees well. Group P5 fungicide. 4-hr reentry. O
- Warhammer at 4 to 12 oz/A. There is no reentry interval or preharvest interval.
Forecasting A model, XanthoCast, can determine blight risk. Temperatures during periods of leaf wetness are measured and a daily index is calculated. A sliding 7-day accumulation of the daily index is calculated. Estimates of inoculum levels can be done the year before in June (for California) by examining 10 trees where Low disease risk = less than 50 total blighted nuts per tree; moderate disease risk = 50 to 150 blighted nuts per tree; and high disease risk = 150 blighted nuts per tree or more.
Biological control
- Blossom Protect (Aureobasidium pullulans) at 1.25 lb/A. Not compatible with many pesticides or additives, a compatibility list is available. 4-hr reentry. O
- DoubleNickel 55 (Bacillus amyloliquefaciens strain D747) at 0.25 to 3 lb/A. Unknown efficacy. Group BM02 fungicide. 4-hr reentry. O
- Serenade Garden Disease Control Concentrate at 2 to 4 fl oz/gal water. Effectiveness alone is unknown in the Pacific Northwest. H O
Reference Lindow, S., Olson, W., and Buckner, R. 2014. Colonization of dormant walnut buds by Xanthomonas arboricola pv. juglandis is predictive of subsequent disease. Phytopathology 104:1163-1174.