Causes and Control Methods of Black Spot Disease in Napa Cabbage

Napa cabbage is one of the common vegetables, and quite a few farmers grow it. During the actual growing process, black spots on the leaves can easily appear, and they are very likely caused by napa cabbage black spot disease. What causes this disease? How can it be prevented and controlled?

1. What causes napa cabbage black spot disease?

Alternaria brassicae, a fungus of the subphylum Deuteromycotina, is an anamorphic fungus. The conidiophores of this fungus are olive-brown and not often branched. Conidia are borne singly; the conidial body has 5 to 12 transverse septa and several longitudinal septa, is grayish olive-brown, and measures 33 to 147 × 9 to 33 microns; the beak has 1 to 6 transverse septa and measures (27 to 111) microns × (4.9 to 7.4) microns, tapering gradually from the conidial body to the beak. In addition, Alternaria brassicicola and Alternaria raphani may also be seen in certain seasons or regions.

Damage to cabbage usually starts from the outer leaves. At first, water-soaked light brown round spots appear, which later become dark brown spots with concentric rings. When humidity is high, black mold-like growth forms on the lesions. The lesions on stems and leaf stalks are dark brown, spindle-shaped, slightly sunken, and ringed, and the diseased parts rot slightly under high humidity. When the disease is severe, diseased leaves wither or drop off.

2. How to prevent and control napa cabbage black spot disease?

1. Seed treatment

The pathogen of black spot disease can attach to napa cabbage seeds and directly infect seedlings. Therefore, before sowing napa cabbage, seeds can be dressed with 50% thiram wettable powder at 0.4% of seed weight, or with 50% iprodione wettable powder at 0.2% to 0.3% of seed weight.

2. Control field humidity

During the disease-prone period, high field humidity leads to severe disease. ① Thin seedlings at the right time. According to factors such as the variety characteristics of napa cabbage and soil fertility, thin seedlings at the right time with an appropriate plant spacing (in the Taiyuan area, no later than early September) to improve field ventilation. ② Water reasonably. During the disease-prone period, water timely and appropriately by bed according to “the sky, the soil, and the growth of the napa cabbage”, and avoid watering followed by rainy weather. Standing water in the field should be drained as early as possible.

3. Foliar fertilization

If the plants are growing weakly, in addition to strengthening water and fertilizer management, 0.2% potassium dihydrogen phosphate or 1% red (white) sugar can be added to the spray solution to supplement foliar nutrients.

4. Spray pesticide early

After the black spot pathogen invades napa cabbage leaves, lesions can form in 3 to 5 days, and black mold-like growth can emerge on the lesions and then invade the napa cabbage again. Therefore, when lesions appear on the lower leaves of napa cabbage, spraying can begin, using 75% chlorothalonil wettable powder at 600 times dilution, or 50% iprodione wettable powder at 1500 times dilution, or 50% thiram wettable powder at 500 times dilution, or 64% oxadixyl-mancozeb wettable powder at 500 times dilution, spraying once every 7 to 10 days for 3 to 5 consecutive sprays, with 50 kg of spray solution per 667 sq m each time.

5. Precautions

Disease-prone plots: land continuously cropped or adjacent-cropped with cruciferous vegetables, land sown without seed treatment, land sown too early, land with high field plant density, land with insufficient base fertilizer and topdressing, land with weak plant growth, and land irrigated by flooding are prone to the disease.

Susceptible vegetables: besides napa cabbage, radish, cabbage, rapeseed (pak choi or green cabbage), cauliflower, etc. are also easily infected by this disease, and control measures can refer to those for napa cabbage.