The following is excellent information on common diseases of mums. With the wet weather, growers should be watching for these diseases.
The primary diseases of garden mums are bacterial leaf spot, Fusarium wilt, Pythium root rot, and Rhizoctonia stem rot and foliar blight. Bacterial leaf spot is caused by Pseudomonas cichorii which is often more prevalent following periods of heavy summer rains.
Bacterial leaf spot development is favored by moisture, high humidity, high temperatures, and/or use of susceptible cultivars. Symptoms include dark-brown to black irregularly-shaped spots that can cover half the leaf. Small spots expand to large lesions under wet conditions. The disease typically begins with the lower leaves and will spread upward. For control, avoid growing cultivars that have been destroyed by bacterial leaf spot in the past: cultivars vary immensely in their susceptibility. Copper sprays may reduce disease spread, but use overhead irrigation as little as possible, and irrigate early in the day so that foliage dries off as quickly as possible.
Fusarium wilt is caused by Fusarium oxysporum f. sp. chrysanthemi. It may be received with mum cuttings or picked up by the roots of the growing crop if the area has been contaminated in previous years. Some cultivars may show no symptoms, others may exhibit yellowing or stunting, and some will be severely wilted and die. To minimize Fusarium wilt losses, avoid growing cultivars that have succumbed to the disease in your experience. Drenches with thiophanate-methyl may be helpful. A pH above 6.0 and nitrate rather than ammonium forms of nitrogen will reduce Fusarium wilt losses, but may create other cultural problems.
Pythium aphanidermatum is the species of Pythium most commonly causing root rot losses to mums in recent years. The pathogen thrives in hot, summer weather. Avoid puddling around containers, deep planting or overwatering. Provide even moisture and avoid high EC values: any root stress may open the plant up to Pythium attack. For control (New York recommendations), utilize etridiazole-containing fungicides (Truban, Terrazole, Banrol) at a monthly interval. An occasional treatment with mefenoxam (e.g. SubdueMAXX) is advisable in rotation with the etridiazole, but beware of using mefenoxam exclusively because of widespread fungicide resistance issues with that active ingredient. 5
Rhizoctonia solani is occasionally a problem either as a stem rot or a foliar disease agent. Affected plants will show wilting or tan-to-brown lesions on leaves, depending on the nature of the Rhizoctonia attack. Practice careful sanitation to keep Rhizoctonia under control: do not allow contact between the mum crop and field soil, which is a source of inoculum. A number of fungicides are effective against Rhizoctonia, including thiophanate-methyl (in 3336, 6672, Banrot), strobilurins (e.g. Heritage, Insignia, Compass) and PCNB (Terraclor).
Information from 'Optimizing Garden Mum Production" by Dr. Mark Bridgen, Professor,
Cornell University http://www.canadiangreenhouseconference.com/talks/2008/2008-TK-Bridgen.pdf
Other diseases we see on garden mums in Delaware are Septoria leaf spot which can be controlled with protectant fungicide sprays if started early and foliar nematodes which can be very difficult to control if present and requires the use of systemic insecticides. Gordon Johnson, Extension Horticulture Agent, UD, Kent County