How to scientifically control bacterial leaf spot of snow peas
The snow pea (scientific name: Pisum sativum var. saccharatum) belongs to the genus Pisum in the Fabaceae family. It is an annual twining herbaceous plant, 90-180 cm tall. It is grown mainly for its tender pods. It originated along the Mediterranean coast and in western Asia. Snow pea tender pods are crisp and fragrant, with high nutritional value. Cooked dry snow peas provide twice the solids, calories, carbohydrates and protein of cooked green snow peas, but green snow peas contain more vitamin A and vitamin C than dry snow peas.
The snow pea, also known as the soft-podded pea, has a minimum germination temperature of 4-6°C, with an optimum temperature of 16-18°C; when the temperature is above 25°C, the germination and emergence rate is low. Seedlings have the strongest tolerance to low temperatures, enduring temperatures as low as -4°C. The suitable temperature during the growing period is 9-23°C; the suitable temperature during the flowering period is 15-18°C; above 25°C, fertilization is poor, few pods are set, quality declines and yields are low. Snow peas require relatively long daylight and strong light. If the weather is overcast during the growing period or field ventilation and light penetration are poor, the plants grow thin and weak, and tender pod yields drop sharply. Snow peas have a fairly wide adaptability to soil, but loose soil with more organic matter and a pH of 6.0-7.2 is suitable. Snow pea rhizobia can fix nitrogen in the soil, but attention should be paid to applying phosphorus and potassium fertilizers. Throughout the growing period, relatively humid air and soil moisture are required. In particular, dry air during the flowering and pod-setting period often causes flower and pod drop; prolonged high temperatures and drought will cause the fibrous tissue of the pods to harden prematurely, leading to premature maturity and reduced quality and yield.
Bacterial leaf spot of snow peas is one of the main diseases of snow peas, and farmers must pay attention to it. Below, let us look at how to scientifically control bacterial leaf spot of snow peas.
Symptoms of bacterial leaf spot of snow peas: also known as Pseudomonas stem blight or stem rot, it damages stems, pods and leaves. When seedlings are infected during the seedling stage, seedlings from infected seeds become diseased; when older plant leaves are infected, the diseased parts are water-soaked, round to angular purple spots, semi-transparent; when humidity is high, white to cream-colored bacterial ooze appears on the underside of the leaves; under dry conditions, a shiny film forms, and the leaf spots dry up and become papery; when the stem is infected, brown streaks first appear; when the pedicel is infected, it can spread from the pedicel to the floral organs, causing flowers to wilt and young pods to shrivel and rot; when pods are infected, the spots are nearly round and slightly sunken, initially dark green, later turning yellowish-brown, with bacterial ooze, 3-5 mm in diameter.
Pathogen of bacterial leaf spot of snow peas: Pseudomonas syringae pv. pisi (Sackett) Young. et al., called Pseudomonas syringae pv. pisi, belongs to bacteria. The bacterial cells are rod-shaped, with one to several flagella at the pole, no spores, no capsule. Gram stain negative, aerobic. Size 0.58-0.82 × 1.11-3.28 microns. On nutrient peptone agar plates, colonies are grayish-white, round and slightly convex, with wavy edges. The optimum growth temperature for the bacteria is 28-30°C, maximum 35-36°C, minimum 0°C, and they are killed at 50°C in 10 minutes.
Transmission routes and disease conditions of bacterial leaf spot of snow peas: the pathogenic bacteria overwinter in snow pea and broad bean seeds, becoming the main primary infection source the following year. Excessive plant growth, failure to drain promptly after rain, and excessive fertilization make the disease more likely to occur. In production, when low-temperature stress occurs, especially after freezing damage, the disease suddenly appears and spreads rapidly. It is prone to occur during off-season cultivation.
Scientific control methods for bacterial leaf spot of snow peas:
1) Establish disease-free seed production fields and collect seeds from disease-free plants.
2) Seed disinfection. Dress seeds with 50% thiophanate-methyl wettable powder at 0.3% of seed weight. Hot water seed soaking can also be used: first pre-soak the seeds in cold water for 4-5 hours, then transfer them to warm water at 50°C for 5 minutes, then transfer them to cool water to cool, dry them and sow.
3) Avoid planting snow peas in low-lying wet land. Use high beds or ridged cultivation, pay attention to ventilation and light penetration, drain promptly after rain, and prevent moisture retention.
4) At the early stage of the disease, spray 72% agricultural streptomycin sulfate at 4000 times dilution, or 27% copper hydroxide suspension at 600 times dilution, 30% basic copper sulfate suspension at 400-500 times dilution, or 47% Kasumin wettable powder at 800 times dilution. Stop applying the pesticide 5 days before harvest.