Symptoms and Control Methods of Smut Disease in the Medicinal Plant Sagittaria

Sagittaria, scientific name Sagittaria trifolia L., commonly known as arrowhead, belongs to the family Alismataceae. The whole plant and corms are used as medicine. It has the effects of clearing heat and stopping bleeding, detoxifying and reducing swelling, and dissipating nodules. It is also a vegetable for autumn and winter. It is mostly planted in rice fields after the rice harvest. It is distributed in all provinces and regions of northern and southern China.

Symptoms mainly damage leaves, petioles, flowers and corms. On the leaves, yellow to orange-yellow, slightly raised blister spots of varying sizes first appear, and later the blister spots wither and yellow and rupture, releasing small black powder particles, which are the spore masses of the pathogen. Some diseased leaves are deformed and twisted. The petioles develop spherical tumors or black streaks. When flowers are damaged, the ovary turns dark brown. When corms are damaged, the disease mostly occurs at the junction of the plant base and the stolon, causing the stem skin to crack. When the leaf incidence rate of the plants reaches more than 60%, the yield is reduced by 20%-30%.

Pathogen Doassansiopsis horiana (P. Henn.) Shen, called Sagittaria false spore smut, belongs to the Basidiomycotina fungi. The spore masses are arranged in one layer, subglobose to oval, size 77-218 x 67-188 (μm). The outer layer and center of the spore mass are sterile cells, and the middle is the spore layer. The outer layer is composed of 3-4 layers of colorless polygonal cells, and the cells in the central part have thin walls and are pale olive-brown. The spores are tightly joined, long and polygonal, smooth, reddish-brown, size 10-17 x 7-10 (μm).

Transmission routes and disease conditions The pathogen overwinters as spore masses in the soil with diseased residues or attached to seed corms. In the following spring, when the monthly average temperature is higher than 15°C, the spore masses germinate and produce basidiospores, which spread through airflow, rainwater or field water for primary infection, and later the diseased parts produce spores for multiple repeated infections. Varietal resistance differs; fields with excessive or one-sided nitrogen fertilizer application, where plants grow tender and green, and continuous cropping fields develop the disease earlier and more severely; high temperature and high humidity are the main factors for disease occurrence; excessive dense planting leads to severe disease.

Control methods (1) At harvest, select seed corms from disease-free fields. (2) Replace with disease-resistant varieties according to local conditions. (3) Implement crop rotation and rational dense planting. (4) Do a good job of field sanitation, and especially after harvest thoroughly collect diseased residues and burn them in a centralized manner. (5) Strengthen water management, avoid long-term deep irrigation, and keep alternating dry and wet conditions to promote root development and enhance plant resistance. (6) In the early stage of disease occurrence, spray as soon as possible with 15% triadimefon wettable powder 1000x dilution or 80% antibiotic 402 emulsifiable concentrate 1500x dilution, 25% carbendazim wettable powder 600x dilution plus 75% chlorothalonil wettable powder 600x dilution, 50% thiram wettable powder 500x dilution, 40% sulfur-carbendazim suspension concentrate 500x dilution, 1:1.5:200-250 Bordeaux mixture, once every about 10 days, for 2-3 consecutive treatments.