How to prevent and control bacterial leaf spot in cantaloupe cultivation

Bacterial leaf spot is an important disease of cantaloupe, and it can occur in both protected and open-field cultivation. In severely affected fields or greenhouses, the diseased plant rate can reach over 80%, and severely diseased plants die from the disease, significantly affecting production.

Symptoms mainly affect the leaves, and in severe cases also infect the stems, vines, and fruits. When leaves are infected, the initial symptoms are dark green, oil-soaked small spots, which later expand into nearly round to irregular grayish brown to yellowish brown spots, often surrounded by a water-soaked dark green halo, and finally the lesions become oil-soaked dark brown and necrotic. In severe cases, the lesions on the leaves interconnect and merge into patches, causing leaf necrosis within a short period. When stems and vines are infected, they appear dark green and oil-soaked, and in severe cases crack and exude gum, and are prone to rot under high humidity. When fruits are infected, initially water-soaked greenish brown small spots usually appear on the fruit surface, surrounded by dark green, and the lesions merge to form yellowish brown to brown necrotic large spots, which readily crack from the diseased area and finally rot.

The pathogen Pseudomonas syringae pv. Lachrymans (Smith & Bryan) Young et al. is called Pseudomonas syringae pv. lachrymans, a pathovar of Pseudomonas syringae, and belongs to the bacteria. The bacterial cells are short rod-shaped, can occur in chains, and measure 0.7-0.9μm×1.4-2.0μm, with 1-5 polar flagella, a capsule, and no spores. Gram stain negative, aerobic. On nutrient agar medium, the colonies are white, nearly round, flat, slightly raised in the center, opaque, with concentric rings, with a thin and transparent ring at the edge, and radial fine hair-like structures at the colony edge.

Disease characteristics: The pathogen overwinters in seeds or with diseased residues in the soil. It invades through wounds or stomata, water pores, and lenticels. After disease onset, it spreads through rain, watering, and insects, and the disease is closely related to dew formation or rain. The pathogen grows at temperatures of 1-35°C, with an optimum development temperature of 20-28°C, stops growing at 39°C, and is killed at 49-50°C. High air humidity, or rainy conditions, or nighttime dew formation are favorable for disease onset.

Control methods

(1) Select and plant disease-resistant varieties according to local conditions.

(2) Strengthen cultivation management.

Carry out crop rotation with non-cucurbit crops for more than 2 years; use disease-free soil for seedling raising and transplant at the appropriate time; apply fertilizer scientifically, increase the application of phosphorus and potassium fertilizers, and improve plant disease resistance; water reasonably, prevent flood irrigation, and in protected cultivation pay attention to ventilation and dehumidification, shorten the time of dew on plant surfaces, pay attention to carrying out farm operations after the dew has dried, and control field pests in time; promptly remove diseased plants found in the field, and after pulling up the vines thoroughly remove diseased residues and fallen leaves to reduce sources of infection.

(3) Seed treatment.

Select disease-free seeds, soak the seeds in warm water at 50-52°C for 30 min before sowing, then sprout and sow, or use 47% Kasumin wettable powder at 0.3% of the seed weight for seed dressing.

(4) Chemical control.

For chemical control at the early stage of disease onset, 5% Kasumin dust at 15kg/hm2 can be used for dusting. It can also be sprayed with 47% Kasumin wettable powder at 600 times dilution, or 77% Kocide wettable powder at 500 times dilution, or 25% dithianon plus basic copper chloride aqueous solution at 500 times dilution, or 25% thiacloprid at 300 times dilution, or streptomycin at 5000 times dilution.

Commonly used agents include Kasumin, Kocide, dithianon, thiacloprid, and streptomycin.