How to prevent and control bacterial leaf spot in primroses
Symptoms On the common primrose (Primula malacoides), leaves and flower receptacles are affected. Initially, water-soaked irregular lesions form along the leaf veins, which later turn yellow, then the lesions turn brown, enlarge, and marginal blight symptoms occur. In severe cases, the lower leaves die.
On the polyanthus primrose, dark water-soaked spots initially form on the leaves, which later turn light brown with a distinct yellow halo around them.
The pathogen Pseudomonas syringae pv. primulae (Arke et Gardner) Young, Dye et Wilkie is known as the primrose pathovar of Pseudomonas syringae. Generally, the common primrose, polyanthus primrose, alpine primrose, and Primula malacoides are relatively susceptible, while Primula kewensis and Primula denticulata have strong disease resistance.
Control methods
1. Strengthen cultivation management: cultivate disease-free seedlings, and use new disease-free soil for raising seedlings in seedbeds; ventilate protected cultivation areas in a timely manner to reduce humidity in sheds and rooms, control watering after disease onset, promote root development and enhance disease resistance; in open fields, adopt high-ridge mulched cultivation, level the land, and improve drainage and irrigation facilities; after the ornamental period ends, remove diseased plant residues and turn and sun-dry the soil, etc. These measures have a certain effect on reducing disease incidence and lessening damage.
2. Chemical control: 50% DT (cupric sulfate-basic copper sulfate) wettable powder at 500 times dilution has a special effect on bacterial leaf spot, and generally cannot be mixed with other pesticides. 60% DTM (cupric sulfate-phosethyl-Al) wettable powder at 500 times dilution, or 77% Kocide wettable microgranules at 400 times dilution.
Streptomycin sulfate or 72% agricultural streptomycin wettable powder at 4000 times dilution can also be used, or new phytomycin at 200-250 ppm.