Occurrence Patterns and Control Methods of Potato Blackleg in Heilongjiang Province

Blackleg occurs to varying degrees in all potato-growing areas. The plant infection rate ranges from 2%-5% in mild cases to 40%-50% in severe cases, often causing missing plants, broken rows, and seed potato rot in the field. In recent years, the damage caused by this disease in our province has shown a trend of increasing year by year, greatly reducing the quality, yield, and marketable potato rate of potatoes. To effectively prevent this disease, it is necessary to accurately identify its symptoms, clearly understand the occurrence patterns, and master integrated control techniques.

1 Symptom Identification

1.1 Disease Symptoms Blackleg can occur throughout the entire growth period, mainly affecting stems and tubers.

(1) Symptoms when aboveground parts are infected: Plants are susceptible to infection when they reach a height of 15-18 cm. Leaves show chlorosis and yellowing and curl upward, internodes shorten, and growth vigor weakens. Diseased plants are easily pulled from the soil, the stems turn black and often split open automatically, exuding foul-smelling mucus. The stem base often carries rotting mother tuber material, ultimately causing the plant to wilt and die. When disease occurs, brown vascular bundles can be seen in the cross-section of the stem. When the disease develops slowly, the plant gradually withers, the tuber-bearing zone moves upward, and aerial tubers are easily formed.

(2) Symptoms when underground parts are infected: A. When seed potatoes are infected, they either do not sprout or gradually rot in the soil after just beginning to sprout and fail to emerge, causing missing plants and broken rows; B. When tubers are infected, the disease starts from the navel end and spreads radially toward the pith. The affected area appears dark brown or black. Cross-sectional examination shows vascular bundles as dark brown dots or short lines. When squeezed by hand, the skin and flesh do not separate. In severe cases, the middle of the tuber rots into a hollow cavity.

1.2 Cause of Disease This disease is caused by infection with the blackleg variety of Erwinia carotovora subsp. atroseptica. This bacterium belongs to the genus Erwinia and is a low-temperature type of soft rot bacterium. It is not tolerant of sunlight and drying; most are killed after 2 h of exposure to sunlight.

1.3 Conditions for Disease Development (1) During cellar storage, poor ventilation and high temperature and high humidity inside the cellar favor bacterial reproduction and damage, often causing massive tuber rot; (2) When cutting seed potatoes, if the cutting knife is not sterilized and the cut pieces are piled together afterward, this is unfavorable for the rapid formation of a cork layer on the cut surface, increasing the disease incidence; (3) In plots with heavy clay soil and poor drainage, low soil temperature slows plant growth, which is unfavorable for the formation of the host cork layer and reduces the ability to resist invasion. At the same time, high soil water content favors bacterial reproduction, spread, and invasion, so the disease is severe.

2 Infection Pathways

Potato blackleg is mainly spread by bacteria carried in diseased tissue and through wound infection. There are two transmission pathways: First, diseased tissue remains directly in the field. Some symptoms of potato blackleg are not easily observed with the naked eye in the field, but they still cause infection and yield reduction, mainly because the pathogen survives in the diseased tissue before it rots. In the field, bacteria enter through wounds via irrigation water, rainwater, mist, machinery, weeds, or insect vectors, infect the buds of tubers along the vascular bundles, and as the plant grows, further invade roots, stems, stolons, and newly formed tubers. They spread along the vascular bundles in all directions, invade the intercellular spaces of nearby parenchyma tissue, secrete pectinase to dissolve the middle lamella of cell walls, causing cells to separate, tissue to disintegrate, and a rotting appearance. Second, diseased tissue enters farmyard manure through various channels and is then applied to the field.

3 Integrated Control

Based on the infection pathways of potato blackleg, two main control methods have been summarized: biological control and chemical control.

3.1 Biological Control

3.2 Chemical Control

(1) Plant disease-free seed potatoes, practice rational crop rotation, and sow whole tubers;

(2) Avoid leaving potato dead branches, fallen leaves, and root tissue in the field. Avoid applying farmyard manure containing potato residues to potato plots. Farmyard manure containing potato residues should be used after full decomposition. This is extremely important for controlling the disease.

(3) Select disease-resistant varieties, such as Kangbing No. 1;

(4) Blackleg is one of the main diseases causing cellar rot. Therefore, proper cellar storage is an issue that cannot be ignored.

3.2.1 Chemical Treatment (1) Soak the cutting knife in 5% carbolic acid or boiling 5% salt water for disinfection, then cut the seed potatoes; (2) Disinfect the cutting knife with 0.1% mercuric chloride or 75% alcohol or 0.1% streptomycin sulfate, then cut the seed potatoes; (3) Air-dry the cut seed pieces promptly so that the cut surface becomes suberized as quickly as possible before planting; or mix the seed pieces with plant ash, calcium peroxide, etc., before planting; or soak the seed pieces in 500 ppm streptomycin sulfate and plant after drying.

3.2.2 Field Control Promptly remove diseased plants by hand, and at the same time spray chemicals at the sites where diseased plants were removed to prevent the disease from spreading. If diseased plants are found in the field, dig them out promptly and destroy them in a centralized manner. Sprinkle a small amount of slaked lime in and around the diseased planting holes, and use 72% streptomycin soluble powder at 4000 times dilution, or 25% copper ammonia complex aqueous solution at 500 times dilution, or 50% DT (cupric dialkyldithiocarbamate) wettable powder, or 95% CT fungicide aqueous solution (copper acetate) at 500 times dilution, or Dexon at 500-1000 times dilution, spraying once every 7-10 days for 2-3 consecutive sprays. Alternatively, 1000 ppm streptomycin sulfate can be injected into mildly affected plants, with 3-4 ml of solution injected per plant each time.