Detailed Guide to Controlling Several Major Diseases of Alliums

The main diseases of alliums include downy mildew, rust, and purple blotch. In recent years, the occurrence and damage of these diseases have tended to worsen in various regions, and this is especially pronounced under protected cultivation, greatly affecting the yield and quality of alliums. Below is an introduction to the main symptoms, disease conditions, and control methods of these 3 diseases.

1. Downy mildew

(1) Main symptoms. Downy mildew is an important disease of Welsh onion and bulb onion. In addition to damaging Welsh onion and bulb onion, it also damages garlic, Chinese chives, and others. Downy mildew mainly infects leaves and flower stalks. On the yellow-green lesions, a layer of white mold-like material grows; this is the mycelium and sporangia of the pathogen. In the later stage, the diseased part is colonized saprophytically by other fungi, producing black mold.

(2) Disease conditions. The occurrence of downy mildew favors low-temperature and high-humidity climatic conditions. The optimum temperature for disease occurrence is 15°C. Under such temperature conditions, the incubation period of the pathogen is 5-10 days. In spring and early summer, if there is warm daytime weather, cool nights, and continuous cloudy and rainy weather, disease occurrence is favored, especially during continuous heavy foggy days, when downy mildew epidemics often occur. In addition, low-lying fields, heavy clay soil, flood irrigation, excessive dense planting, and poorly growing plots also have severe disease occurrence.

(3) Control methods. Adhere to rational crop rotation, carrying out rotation with non-allium and non-garlic crops for more than 2-3 years. On the premise of not affecting growth, reduce field humidity as much as possible, strengthen plant vigor, and improve disease resistance. Strengthen chemical control. Spraying for protection should begin from the early stage of disease occurrence. Commonly used chemicals include: 72% DuPont Curzate wettable powder at 800 times dilution, or 64% Sandofan wettable powder at 800 times dilution, or 58% Ridomil MZ wettable powder at 700 times dilution, or 70% fosetyl-Al manganese zinc wettable powder at 500 times dilution. To improve the adhesion of the spray solution, 500 times diluted laundry detergent solution may be mixed in for spraying, once every 7-10 days, for 3-4 consecutive sprays; if rainy weather occurs, shorten the spraying interval. During the season when downy mildew is epidemic, foggy weather can worsen the disease, so immediately after the fog clears, spray once with clean water to prevent and control an outbreak of the disease.

2. Purple blotch

(1) Main symptoms. On Welsh onion, purple blotch mainly infects leaves and flower stalks; on bulb onion, it can also infect bulbs, causing soft rot and shriveling. In addition to alliums, it can also damage garlic. In the early stage of disease occurrence, water-soaked white spots appear. The lesions expand quickly, rapidly forming sunken spindle-shaped spots 1-3 cm wide and 2-4 cm long, first light brown, then turning brown to bluish purple, with a yellow halo around them. Afterward, some gradually fade and form concentric rings, and under high humidity, dark brown sooty mold-like growth appears on the surface of the spots. If the lesions expand around the leaf or flower stalk, they can cause it to break at the lesion. Bulbs are mostly affected at the neck, where the diseased part shrivels and turns light red or yellow, and mold-like growth also occurs under humid conditions. The characteristic of this disease is that the lesions are spindle-shaped, slender at the upper and lower ends, and darker in color, and complete leaf death rarely occurs, which can be used to distinguish it from downy mildew.

(2) Disease conditions. In southern regions, conidia mostly spread and cause damage on allium crops in repeated cycles. In cold northern regions, mycelium overwinters in the host or on diseased residues, then produces conidia, which spread by airflow and rain and invade through stomata and wounds. The disease conditions are warm and humid weather; below 12°C, the disease does not occur. The wounds caused by onion thrips piercing and sucking are an important route for pathogen invasion. Therefore, when thrips occur severely, disease occurrence can be worsened.

(3) Control methods. Carry out 3-4 years of crop rotation with non-allium and non-garlic crops. Because the pathogen can attach to seeds and spread, seed disinfection should be carefully done before sowing. Seed treatment can be carried out with 50% thiram, 50% carbendazim, or 50% thiophanate-methyl wettable powder at 0.4% of seed weight. Carry out chemical control promptly. In the early stage of disease occurrence, spray 75% chlorothalonil wettable powder, 64% Sandofan wettable powder, 70% mancozeb at 700 times dilution, or 40% difenoconazole wettable powder, 58% metalaxyl mancozeb mixed with water at 500 times dilution, or 50% iprodione wettable powder mixed with water at 1500 times dilution. If onion thrips are damaging at the same time, any one of the above pesticides may be mixed with 80% dichlorvos emulsifiable concentrate or 90% trichlorfon crystal at 1000 times dilution, or 2.5% deltamethrin or 20% fenvalerate emulsifiable concentrate at 3000 times dilution for spraying, to also control onion thrips.

3. Onion rust

(1) Main symptoms. Onion rust not only damages Welsh onion and bulb onion, but also damages garlic and Chinese chives. The small yellow spots produced on diseased leaves are the uredinia of the rust pathogen, and the orange-yellow powder inside them is its urediniospores. In the later stage, some dark brown small spots grow again on the diseased part; these are the telia of the pathogen, containing thousands upon thousands of teliospores.

(2) Disease conditions. The occurrence of rust favors low-temperature and high-humidity climatic conditions. The optimum temperature for spore germination is 9-18%, and the incubation period is 10 days. When the temperature is above 24°C, disease occurrence is affected. Therefore, in spring and autumn, when there is continuous cloudy and rainy weather and the temperature is relatively low, disease occurrence is severe. If the winter is warm and humid, it favors the overwintering of spores, and rust will be severe in the following spring; in summer, if it is cool and rainy, then disease occurrence in autumn will be severe. Insufficient field fertility and poor plant growth are also factors contributing to severe disease occurrence.

(3) Control methods. The main methods for controlling rust are: improve soil fertility, apply more phosphorus and potassium fertilizers, and strengthen the disease resistance of plants; for severely affected fields, harvest early and avoid planting allium and garlic vegetables nearby; for greenhouse cultivation, pay attention to heat preservation and dehumidification, and control the conditions for disease occurrence. In the early stage of disease occurrence, spray promptly for control. The chemicals used include: 25% triadimefon emulsifiable concentrate at 800 times dilution, or 12.5% diniconazole wettable powder at 1500 times dilution for spraying, once every 10 days, for 2-3 consecutive sprays.