Roselle wilt disease strikes easily in the rainy season: master 4 major control methods

Taitung is the main production area of roselle in the country. According to field pest and disease surveys conducted by the Taitung District Agricultural Research and Extension Station of the Council of Agriculture in recent years, roselle wilt disease frequently occurs in roselle fields in Taitung, with the disease rate in some fields exceeding 80%, causing severe losses to farmers. To prevent the disease from spreading, the Taitung Station urges farmers to strengthen field inspections and adopt appropriate control measures to reduce damage.

The Taitung District Agricultural Research and Extension Station points out that roselle wilt disease is caused by plant pathogenic fungi or oomycetes. Once roselle develops wilt disease, the leaves lose water and droop, and the stem near the ground turns brown. In the late stage of wilt disease, the roots of roselle turn brown and rot, and the whole plant becomes dry and withered. Roselle wilt disease mostly shows symptoms after the rainy season, and once it occurs it often spreads rapidly, leaving no time for pesticide control. Because the pathogen of wilt disease is soil-borne, fields with continuous cropping or poor drainage are more prone to the disease.

In fields where roselle wilt disease is severe, most diseased plants have been pulled out and discarded on the field ridges; it is best to remove them from the field and avoid continuous cropping in the next season.

The Taitung Station recommends that farmers control the disease by avoiding continuous cropping, forming ridges and improving drainage, applying phosphorous acid before the rainy season or when only sporadic diseased plants appear, and using chemical pesticides when wilt disease occurs.

1.  Avoid continuous cropping: With soil-borne diseases, when the same crop is planted continuously, the pathogen population tends to accumulate, making the disease more severe. It is recommended to rotate roselle with crops such as millet, Taiwan quinoa or pigeon pea to reduce the density of pathogens in the soil.

2.  Form ridges and improve drainage: Water accumulation in fields accelerates the spread and infection of the disease. In fields with poor drainage, it is recommended to level the land and form ridges before planting roselle to improve drainage.

3.  Apply phosphorous acid for prevention: Phosphorous acid can stimulate plants to enhance their disease resistance, but applying it after the disease has occurred has little effect. Therefore, it is recommended to spray the whole plant with a 500-1,000 times dilution of phosphorous acid before the rainy season or when sporadic diseased plants appear, once every 7 days for 2-3 consecutive times for prevention. It is particularly important to note that phosphorous acid (H3PO3) is a strong acid and must be neutralized with potassium hydroxide (KOH) at a 1:1 equal weight before use. When preparing, the two must first be dissolved in water separately and then mixed together; or dissolve the phosphorous acid first, then slowly add the potassium hydroxide. Never add both to the water at the same time, to avoid generating high heat and causing danger.

4.  Chemical pesticide control: Refer to the recommended pesticides for roselle blight in the Plant Protection Manual, such as 525 g/L dimethomorph water suspension, 60.8% fluopicolide water suspension, 60% metalaxyl-mancozeb water dispersible granules, 85% alkaline copper oxychloride wettable powder, etc. Start applying pesticides at the early stage of wilt disease occurrence, and if necessary apply once every 7 days.

The Taitung District Agricultural Research and Extension Station points out that in recent years wilt disease has seriously affected roselle in Taitung, and reminds farmers to do a good job in disease management to ensure the harvest. If there are any questions about control, farmers can consult the Plant Protection Laboratory of the Taitung Station (telephone 089-325015).