Symptom Identification and Control Methods of Walnut Bacterial Blight
1. Distribution and Occurrence Characteristics Widely distributed in walnut-producing areas such as Hebei, Shandong, Shanxi, Liaoning, Henan, Jiangsu, Zhejiang, Sichuan, Yunnan, Shanxi, and Gansu, with severe incidence in some areas. Infection generally begins in mid-to-late May. The pathogenic bacteria overwinter in the lesions of diseased branch tips or in diseased buds. In the spring of the following year, the bacteria spread by wind and rain splashing onto leaves, fruits, and young branches, causing damage. The pathogen can infect floral organs, so pollen can also carry the bacteria, and insects are also vectors. The bacteria invade through stomata, lenticels, nectaries, and various wounds. Under conditions of sufficient humidity, temperatures within the range of 4-30°C can cause infection of leaves, and 5-27°C can cause infection of fruits. The incubation period varies in different parts: 5-34 days on fruits and 8-18 days on leaves.
2. Pathogen and Damage Symptoms The pathogen of walnut bacterial blight is a yellow-pigmented rod-shaped bacterium (Xanthomonas juglandis), which mainly damages fruits but can also damage leaves, young shoots, and branches. After the fruit is infected, small dark brown oil-soaked spots appear on the green husk, gradually expanding into circular or irregular shapes without obvious margins. In severe cases, the lesions become sunken and deep, and the entire fruit turns black, rots, and drops early. The infection rate is 30%-70%, and in severe cases can exceed 90%. The kernel shrivels, losing 40%-50% of its weight, and nut quality declines.
After leaves are infected, the upper surface turns brown, and the lesions on the underside are light brown, oily and shiny. The lesions are surrounded by a translucent yellow halo. In severe cases, the lesions join together into patches, causing fruit drop. After inflorescences are infected, dark brown water-soaked lesions appear.
The pathogenic bacteria overwinter in diseased branches or shoots. In the spring of the following year, they spread to fruits or leaves via wind, rain, and insects, invading through wounds or natural stomata. The bacteria can also spread with pollen and often occur alongside the activity of the walnut shoot-boring moth (Atrijuglans hetaohei). Rainy summers or humid weather favor bacterial infection, and orchards with high planting density, closed tree canopies, and poor ventilation and light penetration have more severe incidence.
3. Control Methods
(1) Manchurian walnut (Juglans mandshurica) is relatively resistant to bacterial blight and can be used as a rootstock.
(2) Eliminate sources of inoculum. In combination with pruning, cut off diseased branch tips and diseased fruits, and collect fallen fruits from the ground. Burn them in a centralized manner to reduce the source of bacteria in the orchard.
(3) Chemical control. Before bud break, spray 3-5 degrees Baume lime sulfur mixture once to kill overwintering bacteria. During the growing season, spray Bordeaux mixture at 1:0.5:200 (copper sulfate:lime:water), or 50% thiophanate-methyl at 500-800 times dilution. Application method: foliar spray, once before female flowering, once after flowering, and once during the young fruit stage. Spraying 0.4% copper sulfate during the growing season gives good results and is less likely to cause phytotoxicity.