Cause The fungi Septoria chrysanthemi has been reported from Washington and S. leucanthemi has been reported from Oregon and Washington. These fungi generally overwinter in plant debris and are favored by conditions that keep leaves wet for extended periods of time.
Symptoms Small yellow spots, later becoming dark brown or black and enlarging. Lower leaves are generally infected first. The leaves may wither and die but remain attached to the stem for some time.
- Maintain a steady, relatively dry environment by keeping greenhouse humidity below 90%, increasing spacing between plants for good air circulation, and taking care not to splash water on foliage during watering. Heating the greenhouse at night (especially for zero or negative DIF) or venting around sunset may be necessary. Heating in the morning before sunup can also help prevent dew formation as air temperature increases faster than the temperature of plant parts. If using DROP or DIP for size control, do not let humidity rise above 90%.
- Provide good air circulation-space plants apart.
- Remove and destroy affected leaves as soon as noticed.
- Dispose of all plant debris promptly.
- Avoid overhead watering.
Chemical control Many of the chemicals used for Botrytis or rusts will also manage this disease. Use as a foliar spray in conjunction with cultural control method. Tank-mix and/or alternate products from different groups with different modes of action to prevent the buildup of resistant fungi. Limit the use of any one group during crop production.
Reference Trolinger, J.C., McGovern, R.J., Elmer, W.H., Rechcigl, N. A., and Shoemaker, C. M. 2018. Diseases of Chrysanthemum. In McGovern, R.J. and Elmer, W.H. (eds.) Handbook of Florists' Crops Diseases. Springer Int.