Symptoms and Treatment Methods of Cucumber Brown Spot Disease
Cucumber brown spot disease, also known as target spot disease and commonly called “small yellow spots,” has become widespread throughout China in recent years and has become one of the main diseases harming protected cucumber production. The incidence rate is generally 20%-40%, and in severe cases can reach 60%-70%, causing leaf drop, fruit drop, and similar phenomena.What are the symptoms of cucumber brown spot disease? What treatment methods are available?
I. What are the symptoms of cucumber brown spot disease?
The disease mostly begins during the full fruiting stage of cucumber. The middle and lower leaves are affected first, then it spreads upward. In the early stage, small grayish-brown spots appear on the leaf surface and gradually expand into light brown or brown lesions of varying sizes, either round or nearly round with somewhat irregular edges. Most lesions are 8-15 mm in diameter, the small ones 3-5 mm, and the large ones 20-25 mm. In the later stage, the center of the lesion becomes lighter in color, sometimes grayish-white, with grayish-brown edges. When humidity is high, sparse light grayish-brown mold-like growth appears on both the front and back of the lesions. Sometimes the lesions merge, and the leaves turn yellow and withered. In severe cases, elliptical grayish-brown lesions also appear on the stems and leaf stalks.
Seeds can carry the pathogen, though the seed-borne rate is not high. More pathogens can be isolated from pumpkin seeds; therefore, even if cucumber seeds do not carry the pathogen, when grafted onto pumpkin, the pathogen carried by pumpkin seeds can also become the initial source of infection. The pathogen overwinters in the soil and spreads through airflow or rain splash for primary infection. The conidia produced by lesions formed after primary infection spread to the surroundings via wind and rain. Conidia dispersal mostly occurs during the daytime, with 10:00-14:00 being the peak period of spread. When temperature and humidity conditions are suitable, the pathogen invades quickly. The disease is most severe at 25-28°C and saturated relative humidity, and environmental conditions with large day-night temperature differences aggravate the disease. The disease is severe when plants are weak, when nitrogen fertilizer is applied excessively, and when the trace element boron is deficient. Increasing the application of phosphorus and potassium fertilizers can reduce the disease.
II. What treatment methods are available for cucumber brown spot disease?
1. Seed disinfection
When grafting cucumber onto pumpkin, make sure the pumpkin seeds are also pathogen-free. Soak cucumber seeds and the black-seed pumpkin seeds used for grafting in 55°C warm water for 30 minutes. (Three hours before sowing, dress the seeds with live Pseudomonas fluorescens, dry them in the shade, and then sow.
2. Agricultural measures and chemical control
Promptly use Shibeishou high-efficiency biological pesticides for prevention and treatment. Affected fields should be rotated with non-cucurbit crops for more than 2 years. For plots where rotation is not possible, promptly use live Pseudomonas fluorescens to drench the roots and spray the leaves, so as to ensure the elimination of various pests and diseases caused by continuous cropping pathogens. At the same time, live Pseudomonas fluorescens can improve the soil, increase the soil's organic matter content, and kill various harmful pathogens caused by soil-borne diseases. Thoroughly remove diseased plant residues from the field and subsequently deep-plow the soil to reduce the initial source of infection in the field. Apply sufficient base fertilizer, topdress in a timely manner, avoid excessive application of nitrogen fertilizer, increase the application of phosphorus and potassium fertilizers, and apply an appropriate amount of boron fertilizer. Prevent premature aging of cucumber plants, pay attention to ventilation and dehumidification after watering, and remove diseased leaves in the early stage of the disease.