Symptoms and Control Methods of Alternaria Leaf Spot on Impatiens

[Symptoms]

The spots on affected leaves are round to oval, dark brown, slightly zonate, 2-10 mm in diameter, with a slightly darker margin. Some spots may expand and merge into patches, bearing a pale green or olive-black mold-like growth, which consists of the pathogen's conidiophores and conidia. The older leaves in the middle and lower parts of the plant are more susceptible; in severe cases, the diseased leaves shrivel and die.

[Pathogen]

Alternaria tenuis Nees., belonging to Deuteromycotina, genus Alternaria, a fungal pathogen. Conidiophores are erect, brown, with flexuous bends, the apex often enlarged and bearing conidial scars, measuring 5-125 um x 3-6 um. Conidia are produced in chains, beaked, dark brown, with 1-9 transverse septa and 0-6 longitudinal septa, measuring 7-70.5 um, with a beak measuring 1-58.5 um x 1.5-7.5 um.

[Occurrence Pattern]

The pathogen overwinters as mycelium on diseased plant residues. When conditions are suitable in spring of the following year, it produces conidia that cause primary infection. Large numbers of conidia are produced on new lesions and spread by air currents, causing repeated secondary infections. High humidity, frequent rain and heavy dew favor disease development and epidemics. In Northeast China, the peak period of disease occurrence is generally from July to August and continues until early to mid-September.

[Control Methods]

(1) Remove diseased plant residues from nursery beds or potted plants, and bury them deeply or burn them in a centralized manner to reduce the amount of overwintering inoculum; deep-plow the soil or rotate with other flowers. Strengthen cultivation management, apply fertilizer and irrigate reasonably, and improve plant disease resistance. Promptly remove old lower leaves and diseased leaves.

(2) Spray 1% Bordeaux mixture once before disease onset for protection; after disease onset, spray 50% carbendazim or 50% thiophanate-methyl wettable powder at 800-1000 times dilution, or 50% Tuzet at 800 times dilution.