Identification and Control of Leaf Spot Disease on Stem Lettuce

Stem lettuce, also known as yumai cai, ku cai or sheng cai, belongs to the Asteraceae family, Lactuca genus. It is a pointed-leaf type leaf lettuce harvested for its tender shoots and young leaves. The leaves are long and lanceolate, pale green in color, crisp and tender in texture, with an extremely fresh, delicate and fragrant flavor and a distinctive taste. It contains large amounts of vitamin A, vitamin B1, vitamin B2 and abundant calcium, iron and other nutrients, making it a premium choice among vegetables eaten raw, and it is known as the “phoenix tail”. Stem lettuce belongs to the Asteraceae family and is a pointed-leaf type leaf lettuce harvested for its tender shoots and young leaves. Its leaves are long and lanceolate, and its appearance is somewhat like the “head” of celtuce, with slender, flat-spreading leaves and a thin, short stem. From a genetic standpoint, stem lettuce belongs to a variety of leaf lettuce——long-leaf lettuce, which is similar to the familiar sheng cai, hence it is also called niu li sheng cai. It has a pale green color, grows vigorously, has strong disease resistance and adaptability, a crisp and tender texture, and an extremely fresh, delicate and fragrant flavor, earning it the name “phoenix tail”.

Introduction to leaf spot disease on stem lettuce:

Symptoms: The lesions occur on both the upper and lower surfaces of the leaves. On the upper surface, the lesions are circular, nearly circular to irregular in shape, 1-8mm wide, light brown to brown, with a grayish-white center and a yellowish-brown margin. On the lower surface, the lesions are slightly lighter in color.

Pathogen: Cercosporalactucae-sativaeSawada, known as lettuce cercospora, belongs to the fungal kingdom, Deuteromycetes (asexual spore type). Conidiophores and conidia are borne on both leaf surfaces, with no stroma or consisting of only a few brown cells. Conidiophores are solitary or in clusters of 1-12, light brown, unbranched, with 1-4 geniculate bends, conical at the apex, with 0-4 septa, 25-115×4-6(μm). The conidial scars are distinctly thickened, 2-3μm wide. Conidia are needle-shaped to cylindrical, colorless, erect, pointed at the apex, obconical at the base, with multiple septa, 31-117.5×3-5(μm). In addition to infecting lettuce, it also infects Ixeris denticulata, mountain lettuce, long-leaf lettuce and Ixeris dentata.

Disease transmission routes and onset conditions: The fungus overwinters as mycelium and conidial clusters on diseased plant debris. Conidia cause primary infection and reinfection, spreading through air currents and rain splashes. Rainy weather or heavy fog and dew generally favor disease development. Poor plant growth, or excessive nitrogen fertilizer causing overly vigorous growth, will worsen the disease.

Control methods for leaf spot disease on stem lettuce:

(1) Pay attention to field sanitation. Collect diseased leaves and plant debris during harvesting and carry them out of the field to be burned.

(2) Clear drainage ditches to remove waterlogging. Apply well-rotted organic fertilizer and avoid excessive nitrogen fertilizer. Timely spray with natural brassinolide and similar products to promote healthy plant growth and enhance resistance.

(3) At the early stage of disease onset, begin spraying 78% mancozeb•zinc wettable powder at 500 times dilution, or 2.5% fludioxonil suspension concentrate at 1200 times dilution, 50% iprodione wettable powder at 1500 times dilution, or 86.2% cuprous oxide wettable powder at 1000 times dilution, once every 10-15 days, for 2-3 consecutive applications.