What are the control methods for primrose leaf spot disease?

Symptoms The disease occurs on the leaves. The spots are usually 3-4 mm in diameter and brown; in severe cases, the diseased leaves wither and die, causing defoliation. The disease spreads from the lower leaves to the upper leaves.

Pathogen Ramularia primulae Thumen, the primrose Ramularia fungus. It belongs to the Deuteromycotina fungi. The conidiophores emerge from the stomata, growing in clusters of several branches, very short, unbranched, colorless, with slightly curved tips; the conidia are cylindrical or short club-shaped, single-celled or two-celled, colorless, measuring 16-24 x 3-4 micrometers.

Transmission routes and conditions for disease development The fungus overwinters as mycelium or conidiomata on diseased plant debris, and seeds can also carry the fungus, becoming the primary infection source the following year. The conidia produced on the diseased parts spread through airflow and rain splash, and secondary infection is frequent during the growing season. Generally, warm and humid weather and excessive application of nitrogen fertilizer, with plants growing too densely, favor disease development. The disease usually begins in July and becomes epidemic from August to October.

Control methods 1) Select and breed disease-resistant varieties. 2) Strengthen fertilizer and water management, apply more organic fertilizer and phosphorus-potassium fertilizer, and avoid excessive application of nitrogen fertilizer. 3) After the disease first appears at the seedling stage, begin spraying 70% thiophanate-methyl wettable powder at 1000 times dilution plus 75% chlorothalonil wettable powder at 1000 times dilution, or 50% thiophanate-methyl and sulfur suspension concentrate or 36% thiophanate-methyl suspension concentrate at 500-600 times dilution, or Bordeaux mixture at 1:1:100.