Identification and Control of Black Spot Disease in Snow Peas
The snow pea (Pisum sativum var. macrocarpon) belongs to the genus Pisum in the legume family (Fabaceae). It is an annual twining herb growing 90-180 cm tall. The earliest region where snow peas were cultivated was along the Thailand-Myanmar border area approximately 12,000 years ago. The round-bodied type is also called sugar snap pea or honey pea (Sugar snap peas), while the flat-bodied type is called green pea or snow pea (Snow pea). It also has many other names including small winter pea, Huai pea, hemp pea, green small pea, retained pea, golden pea, Hui-Hui pea, wheat pea, wheat bean, Bi pea, Ma Lei, national pea, soft-podded pea, podded pea, sweet pea, lotus-seed pea, green speckled pea, vegetable pea, and many others. It can be used as food. The snow pea, also known as lotus-seed pea or cut pea, belongs to the genus Pisum in the legume family (Leguminosae). It is mainly grown for its tender pods. It originated along the Mediterranean coast and western Asia. Snow pea tender pods are crisp and fragrant with very high nutritional value.
Growth Habits of Snow Peas
Light requirements: Snow peas are long-day plants. Most varieties can flower earlier when the photoperiod is extended and flower later when the photoperiod is shortened, but some early-maturing varieties are not strict about light requirements. Generally, varieties require strong light and longer day length during the pod-setting period, but are not suited to high temperatures, and are best planted during the transition between spring and summer.
High water demand: Snow peas require relatively large amounts of water throughout the entire growing period. During seed germination, if soil moisture is insufficient, the seeds cannot absorb water and swell, which greatly delays emergence. During the seedling stage, they can tolerate a certain degree of drought. If air humidity is too low during the flowering period, it can cause flower and pod drop. If high temperatures and drought occur during the pod-growing period, the pod fibers will harden prematurely, ripening too early and reducing quality and yield. Therefore, throughout the entire growing period of snow peas, there must be an adequate water supply for vigorous growth, large pods, and full seeds, ensuring both quality and quantity. However, they are also not tolerant of waterlogging; if moisture is excessive, seeds easily rot after sowing, roots easily rot during the seedling stage, and diseases easily occur during the growing period.
Soil conditions: Although snow peas are not strict about soil requirements and can be planted in well-drained sandy loam or newly reclaimed land, loose neutral soil containing organic matter is preferable, and soil with a pH of 6.0-7.2 is suitable for growth. When soil acidity is below pH 5.5, diseases easily occur and the pod-setting rate decreases, so lime should be added to improve the soil. Pea roots are deep, slightly drought-tolerant but not moisture-tolerant; poor drainage during sowing or seedling stage easily causes root rot; drought during the flowering period causes poor fertilization, easily forming empty pods or shriveled pods. Snow peas should not be continuously cropped; generally at least 4-5 years of rotation are required.
Nutritional Value of Snow Peas
The tender shoots, tender pods, and seeds of snow peas are tender and fragrant, and are extremely popular for eating. Every 100 g of tender pods contains 71.1-78.3 g water, 14.4-29.8 g carbohydrates, 4.4-10.3 g protein, 0.1-0.6 g fat, 0.15-0.33 mg carotene, and also contains amino acids essential to the human body. Every 100 g of peas contains 7 g protein, 12 g carbohydrates, 33.4 kilojoules of calories, 17 mg calcium, 90 mg phosphorus, 0.6 mg iron, 0.15 mg carotene, 0.54 mg thiamine, 0.09 mg riboflavin, 2.8 mg nicotinic acid, and 14 mg VC. Peas contain a unique phytohemagglutinin, a branching inhibitor, and gibberellin A20, among others; these substances play an important role in enhancing human metabolic function.
Symptoms and Control of Black Spot Disease in Snow Peas
Symptoms of black spot disease in snow peas:
Black spot disease in snow peas mainly damages leaves, stems near the ground, and pods. When leaves are infected, circular to irregular spots initially appear, with the center being dark brown to black, with 2-3 concentric rings, on which many small black granular dots form, namely the pathogen's pycnidia; when stems are infected, streak spots appear on the stems, with the diseased area being dark brown, and the stems and leaves above the stem turn yellow and die; when pods are infected, irregular purple spots initially appear, the diseased area has secretions, brown to dark brown, and after drying becomes scab-like, invading the seeds and causing spots.
Pathogen Ascochyta pinodes (Berk. et Bloxam) Jones, called bean Ascochyta, belongs to the Deuteromycetes fungi. Its sexual state is Mycosphaerella pinodes (Berk. et Bloxam Vestergren), called snow pea spherical cavity fungus, belonging to the Ascomycota fungi. Ascocarps are spherical, 100-140 micrometers in size; asci are cylindrical to elliptical, 58-62 × 12 micrometers in size; ascospores are oval or elliptical, 17.2 × 7.9 micrometers in size. Conidia are colorless, two-celled, 12-17 × 3.5-5.5 micrometers in size, averaging 12.3 × 4.5 micrometers.
Transmission routes and disease conditions of black spot disease in snow peas:
The fungus overwinters as mycelium or conidia within seeds, or as pycnidia with diseased residues on the soil surface. The following year, the pathogen spreads through wind, rain, or irrigation water, and invades through stomata, water pores, or wounds, causing disease. Seed-borne bacteria can spread over long distances with seed transport. When diseased seeds are used for seedling raising, cotyledon infection can be seen during the seedling stage, then it spreads to the true leaves. After disease occurs in the field, conidia are produced on the lesions and spread by wind, rain, or agricultural operations, causing secondary infection.
Control methods for black spot disease in snow peas:
1) Select disease-free pods and thresh separately for seed saving. Before sowing, soak seeds in 56°C warm water for 5 minutes for seed disinfection.
2) Sow at the appropriate time, not too early. Promote high-ridge cultivation, rational fertilization, appropriate close planting, and increased potassium fertilizer application to improve disease resistance.
3) In the early stage of disease, spray 27% copper hydroxide suspension concentrate at 600 times dilution, or in combination with the control of other leaf spot diseases of snow peas, spray early with 75% chlorothalonil wettable powder at 1000 times dilution plus 70% thiophanate-methyl wettable powder at 1000 times dilution, 75% chlorothalonil wettable powder at 1000 times dilution plus 70% mancozeb wettable powder at 1000 times dilution, or 50% thiophanate-methyl • sulfur suspension concentrate at 800 times dilution, once every 10 days or so, for 2-3 consecutive times, taking care to spray evenly and thoroughly. Stop applying the pesticide 7 days before harvest.