Detailed Guide to Identifying and Controlling the Main Diseases of Lilies

Lilies belong to the genus Lilium in the family Liliaceae and are perennial herbaceous plants, also known as wild lilies and medicinal lilies. In recent years, lily cultivation has been seriously threatened by diseases, mainly lily leaf blight, phytophthora blight, bulb basal rot, and viral diseases. These diseases often occur together, worsening the damage. The usual incidence rate is 10%-15%, and in severe cases it can reach 20%-30%.

1. Several main diseases of lilies

(1) Lily leaf blight, also known as lily gray mold, is a commonly occurring disease. It mainly damages leaves, and can also infect stems, buds, and flowers. Leaf spots are round or oval, varying in size, light yellow to light reddish brown, and water-soaked in appearance. Under warm and humid conditions, the spots become thin and brittle, light gray, and semi-transparent, and in severe cases can cause the entire leaf to wither and die; infected stems turn brown or become constricted, causing the plant to collapse; when young plants are infected, the growing points of stems and leaves become soft, rot, and die; infected flower buds initially show small brown spots, which then expand and cause the buds to rot. Under humid conditions, a large amount of gray mold develops on the diseased parts. The pathogen mainly overwinters as sclerotia left in the soil and as mycelium on diseased residues. In spring of the following year, the overwintered mycelium produces gray mold on diseased parts or on sclerotia, and conidia are spread by airflow to cause primary infection. Conidia on diseased parts of field plants are spread by wind and rain, rapidly expanding and spreading. The optimum growth temperature for this fungus is 22-25°C. Generally, in early June, when there is more rainfall, heavy fog and dew, and field RH is 90%, the disease spreads relatively quickly.

(2) Lily viral disease causes infected plants to show mosaic symptoms with mottling of light green and dark green. Severely diseased plants are noticeably dwarfed, with shorter scales, and some develop brown and necrotic spots. According to surveys, the occurrence of viral disease is closely related to the source of the seed stock. Some locally retained seed bulbs have not been virus-free, while another portion introduced from the Netherlands are very expensive but still have severe virus problems, and planted lilies generally have viral disease at over 30%.

(3) Bulb basal rot mainly damages the stem base of lily plants, affecting plant growth and leading to the death of the whole plant. This disease is relatively common in many areas, with an incidence rate generally of 10%-15%, and as high as 10%-30%. It usually begins when the plant is flowering, with the plant base turning dark brown and rotting, leaves drooping and turning yellow, ultimately leading to death. The pathogen overwinters in the soil with diseased residues and becomes the initial infection source. Waterlogged soil, heavy clay soil, application of incompletely decomposed fertilizer, and damage by nematodes and underground pests easily lead to disease occurrence. This fungus requires relatively high temperature and humidity, so the disease often occurs suddenly during the middle and late stages of plant growth, when temperatures rise and after continuous rainy days followed by clearing.

(4) Lily leaf tip blight is relatively common and mainly damages leaf tips. Affected leaf tips turn blackish brown, necrotic, or dry up and continuously spread toward the leaf base. When the middle of the leaf is affected, spindle-shaped or oval spots form, with brown or reddish brown edges and a grayish white center, and many small black dots are scattered on the diseased parts. The pathogen overwinters on diseased residues or diseased plants in the field, and in the second year is spread by wind and rain, invading through wounds or the epidermis and causing disease in the field. This disease begins to occur in June. Plants with poor fertilizer and water management and weak growth are more susceptible.

(5) Lily phytophthora blight, also known as lily foot rot. It is more severe in years with abundant rain, causing stems and leaves to rot and plants to lodge, affecting bulb yield. It mainly damages stems and leaves. On leaves, large water-soaked grayish green to brown spots appear, with no obvious edges. In severe cases, flowers and bulbs show water-soaked brown spots and rot, and under humid weather, sparse white mold grows on the diseased parts. When stems and stem bases are affected, water-soaked brown wrinkled stripe spots initially appear, gradually expanding upward and downward, causing stem rot, plant withering and lodging, and sometimes the top of young leaves and stems withers. The pathogen overwinters in the soil with diseased residue tissues, and in the second year during the Spring Festival, when conditions are suitable, it infects the host and causes disease. When there is abundant rainfall and high air and soil humidity, large numbers of sporangia form on diseased parts and cause secondary infection through rain splash.

2. Control measures

(1) Select healthy, disease-free bulbs, strengthen management during bulb storage, and prevent bulbs from losing water. To prevent viral disease, virus-free bulbs should be planted. First, through seed propagation; second, through shoot tip treatment, taking 0.1-0.2 cm shoot tips for tissue culture to obtain virus-free plantlets. However, virus-free plantlets must be planted in isolation, and aphids must be controlled to prevent natural infection.

(2) Before sowing, soak the seeds for 15 minutes in a 1:500 thiram solution or a 50-fold dilution of 40% formaldehyde.

(3) Strengthen field management. The planting site should have high beds and deep furrows, with level bed surfaces to facilitate drainage. During irrigation, avoid wetting the leaves. Improve ventilation and light conditions, and increase the application of phosphorus and potassium fertilizers to make the plants grow robustly and enhance disease resistance.

(4) Promptly remove diseased plants, thoroughly clear diseased residues from the ground, and burn them in a centralized manner. After removing diseased plants, disinfect with 50% lime milk.

(5) Chemical control. For seedling diseases, after emergence, spray 2-3 times with a 1000-fold dilution of 50% carbendazim wettable powder, and promptly remove diseased seedlings. For phytophthora blight, options include a 2000-fold dilution of 25% metalaxyl, a 300-fold dilution of 40% fosetyl-aluminum, a 1000-fold dilution of 70% dexon original powder, or a 1000-fold dilution of 70% thiophanate-methyl. When spraying, make sure the pesticide reaches the stem base of diseased plants and the surrounding soil.