Symptoms of blackleg disease in tobacco, potato, cabbage and geranium and the pesticides used for its control

Blackleg is a bacterial disease. The pathogen can overwinter on plant residues in the soil or on diseased tubers during storage, but it cannot survive in the soil for too long. It occurs on crops such as tobacco, potato, cabbage, Chinese cabbage and geranium. So what symptoms does blackleg disease have? What pesticide should be used?

Potato blackleg

Potato blackleg is a bacterial disease that infects vascular bundle tissue, and it can occur throughout the entire growth cycle of the potato. Warm and humid conditions favor invasion by the pathogen; especially in years with a lot of rain, potatoes in low-lying areas suffer more severely from blackleg.

Symptom expression

Seed tubers: infected seed tubers rot into a sticky mass, do not sprout, or rot in the soil just after sprouting and fail to emerge.

Seedlings: infected seedlings generally show symptoms at a plant height of 15-18cm; the plants are short, the internodes are shortened, or the leaves curl upward, turn chlorotic and yellow, or the shank turns black and the plant wilts and dies. A cross-section of the stem shows that the three main vascular bundles have turned brown.

Tubers in the field: infected tubers begin at the navel end and spread radially toward the pith; the diseased area is dark brown, and a cross-section shows that the vascular bundles are also dark brown. When pressed by hand, the skin and flesh do not separate. When humidity is high, the tubers turn dark brown, rot and smell foul, which distinguishes this from bacterial wilt.

Transmission routes and patterns

Seed tuber transmission: seed tubers carry the bacteria, while soil generally does not. When cutting seed potatoes, contamination by the cutting knife is the main route of disease spread. The pathogen first spreads and expands through cut seed potato pieces, causing more seed tubers to become diseased, then enters the plant through the vascular bundles or pith, causing disease in the aboveground parts.

Field transmission: in the field, the pathogen can spread through irrigation water, rainwater or insects, invading through wounds and causing disease. Later, the pathogen on diseased plants is transmitted from the aboveground stems through the stolons to newly formed tubers.

Storage transmission: during storage, the pathogen invades healthy tubers through wounds or lenticels through contact between diseased and healthy tubers, causing them to become diseased.

Morphological characteristics

Potato blackleg is a bacterial disease. The pathogen belongs to the genus Erwinia. The bacterial cells are short rod-shaped with bluntly rounded ends, have 4-5 peritrichous flagella, do not form spores, and are Gram-negative. On agar medium the colonies are grayish white and round. The pathogen grows well within the range of 10-38°C, with the optimum at 25-27°C, and loses viability above 45°C.

Control methods

(1) Select disease-resistant varieties.

(2) Select disease-free seed tubers and establish disease-free seed production fields.

(3) Sterilize the cutting knife promptly with alcohol or potassium permanganate when cutting seed potatoes. After cutting, dress the seed pieces with Jiasoumi or Cailong, dry them in the shade, and sow immediately.

(4) Sow early at the proper time, pay attention to drainage, reduce soil moisture, raise soil temperature, and promote early emergence.

(5) Strictly select seed tubers before storing them in the cellar, strengthen management after storage, and keep the cellar temperature at 1-4°C to prevent excessively high cellar temperature and excessive humidity.

(6) Chemical control:

After emergence, for prevention, spray 100 ml of Jiasoumi or Cailong on the leaves; in the early stage of disease, for drip irrigation use 300 ml of Jiasoumi or Cailong as a root drench, and for sprinkler irrigation use 500 ml as a root drench. In the middle and late stages of disease, use 200 g/mu of Jiaruinong for drip irrigation and 300 g/mu for sprinkler irrigation.

Tobacco blackleg

Symptoms

When seedlings become diseased, most first develop black spots on the roots and stems near the soil surface. The diseased area expands upward and downward, extending to all stems, leaves and the main root. The base of the diseased stem becomes constricted and is likely to cause damping-off. In high humidity and rainy conditions, the diseased seedlings rot completely, produce white cottony mold on the surface, and spread rapidly to nearby seedlings, causing tobacco seedlings to rot and die in patches. If the weather is dry, the diseased seedlings dry up and turn dark brown. When seedlings are only mildly affected, the symptoms are not obvious. After tobacco seedlings are transplanted to the field, they are highly prone to disease when exposed to high temperature and high humidity. Most begin to blacken at the junction of root and stem near the soil surface and expand upward, often reaching as high as 0.6-1 chi, hence the name blackleg disease. Sometimes the middle part of the stalk becomes diseased and black, causing the tobacco plant to die; sometimes the stem shows no lesion but the plant wilts and dies. When the diseased plant is pulled up, the main root and branch roots are seen to be blackened and rotted. Although this disease has various symptoms, the common feature is that the leaves of diseased plants gradually turn yellow, droop and wilt from the bottom upward. In the early stage of disease, they can recover after the night passes. After 4-5 days all leaves wilt, and finally the whole plant dies. Because most of the fine roots of diseased plants have rotted, they are very easy to pull up. When the affected stem is cut lengthwise, in addition to the cortex turning black, the pith also turns dark brown and dries and shrinks into overlapping flakes, with sparse white mold growing between the flakes. In rainy and humid weather, white mold also grows on the surface of the diseased tissue.

Control methods

1. Select and plant disease-resistant varieties

2. Strengthen cultivation management. In cultivation, spring tobacco should be developed and transplanting should be advanced so that the infection stage avoids the rainy season. Implement crop rotation of more than three years; build ridges and earth up, open furrows for drainage, and ensure that tobacco fields do not accumulate water and do not flow “excess water”. After ridging, surface runoff should not contact the stem base, so as to reduce opportunities for infection; apply clean fertilizer and keep flowing water from being contaminated by the pathogen. Pulled diseased plants and picked diseased leaves should be centrally disposed of.

3. Chemical control. Based on the three characteristics of the blackleg pathogen - that zoospores move in the surface soil layer to cause secondary infection, that it mainly infects the stem base, and that the infection stages are after transplanting and before budding - chemical control can generally achieve more than 60% control efficacy. This is especially necessary for some high-quality varieties with poor disease resistance. Dexon can be used at 7 liang per mu, mixed evenly with 30-40 jin of dry fine soil, and applied as medicated soil once before closing the hill and once before ridging, scattered around the tobacco plants, followed immediately by covering with soil so that Dexon does not lose effectiveness on exposure to light.

Cabbage blackleg

It mostly occurs in high-temperature, high-humidity areas and seasons, and can be harmed at both the seedling stage and the adult plant stage, causing plant death in severe cases. Besides cabbage, this disease also harms Chinese cabbage, rapeseed, cauliflower, turnip, radish, head cabbage, Chinese kale and celery.

Symptoms

At the seedling stage, light brown lesions form on the cotyledons, true leaves and young stems. The lesions are round or oval, with small black dots scattered on them. The lesions on young stems are slightly sunken, and severely diseased seedlings die quickly. At the adult plant stage, leaf lesions are the same as at the seedling stage, and purple-black streaks form on the main and lateral roots, causing root rot or breaking and falling at the diseased stem.

Control methods

Seed treatment: soak seeds in warm water at 50°C for 20 minutes. Chemical seed dressing: dress seeds with 50% thiram wettable powder, with the pesticide weight being 0.4% of the seed weight. Soil treatment: use 40% thiram wettable powder or 40% quintozene powder at 8-10 grams per square meter, mixed with 30-40 kg of fine dry soil, and scatter it on the seedbed surface at sowing. Change crops and rotate: for heavily affected fields, promptly change crops or rotate with non-cruciferous vegetables; rotation with field crops is better, with an interval of more than 3 years. Chemical control: spray in the early stage of disease, using 75% chlorothalonil wettable powder 600-fold solution, or 40% duosulfur suspension 600-fold solution, or 60% duofu wettable powder 600-fold solution, etc., spraying once every 8-10 days for 2 consecutive sprays.

Geranium blackleg

Symptoms

This disease occurs on both cuttings and adult plants, and cuttings are affected more severely. Rot begins from the stem base and develops upward, causing death. After adult plants become diseased, they turn black and the disease expands upward, leaves fall off, and the whole plant eventually wilts and dies.

Pathogen

Pythium splendens, belonging to the Mastigomycotina. The hyphae are well developed, branched, non-septate, and appear as white cottony fluff when growing vigorously. Sporangia are filiform, round or nearly round, and produce zoospores that cause infection.

Control methods

1. When taking cuttings, branches must be cut from healthy plants and must be inserted into a seedbed free of the pathogen.

2. When transplanting, a new plot must be used or the potting soil changed to prevent transmission of the disease. If necessary, 70% Dexon wettable powder 1000-1500-fold solution can be poured on the soil surface.

Broccoli blackleg

[Symptoms] Also called root rot, black root disease, etc. It can be harmed at both the seedling stage and the adult plant stage. At the seedling stage, grayish white irregular lesions can appear on cotyledons, true leaves or young stems. When the stem base is diseased, it spreads to the roots, forming dark purple streak-like spots. If the stem base cankers severely, diseased plants break easily and dry up. At the adult plant stage, irregular to angular grayish white large lesions form on the leaves, bearing many small black dots, which are the pathogen's pycnidia. When flower stalks and seed pods are diseased, they are similar to those on stems. When seed plants are diseased during storage, the leaf heads dry-rot; when the diseased stem is cut open, the vascular bundles of the diseased root turn black.