See:
Chrysanthemum - Verticillium Wilt
Cause Although not common, the OSU Plant Clinic has found several soilborne organisms associated with chrysanthemum root problems including Fusarium sp. and Pythium sp., and sometimes Phytophthora sp. and Rhizoctonia sp.
Fusarium wilt - Two different fungi can be involved including Fusarium oxysporum f. sp. chrysanthemi (of which there are 3 races with race 1 detected in the USA) and F. o. f. sp. tracheiphila. Conditions that favor plant growth also favor disease development. The fungi survive in plant debris. They can sporulate on infected tissue and/or be moved by fungus gnats and shore flies. Symptoms are more prominent at higher temperatures and can be delayed several weeks at cooler temperatures.
Pythium root rot (and Phytophthora) - These organisms are favored by abundant moisture and can be especially problematic in poorly drained potting media. Pythium usually attacks unrooted cuttings. Pythium spp. can survive in dust, planting media, or soil particles on greenhouse floors, flats, and pots. Wood benches can also be a reservoir. Spread can also occur via movement of infected plants. Damaged plants are more easily infected. These organisms enter roots damaged by high soluble salts, either from over-fertilization or from letting the rooting medium dry out, even for short periods, during propagation.
Rhizoctonia - More of a problem during propagation and thrives in media that is moist and warm.
Symptoms Fusarium wilt - wilting and then death of some portion of the plant usually starts at the bottom of the plant and progresses upward but that can be reversed in some cultivars. One sided wilting or sectoring of the plant can also occur. Vascular discoloration is seen when stems are cut open. A white or pinkish mycelial growth may be found in the center of the plant. Roots do not initially rot but the root system may be smaller and eventually collapse.
Pythium root rot - Wilting, foliar chlorosis, and stunting are common symptoms. Infected plants may be lighter green, smaller in size, and wilt during the heat of the day. Lower leaves turn yellow and drop from the plant. Water-soaked and decayed roots along with dark-brown necrotic stems with a wet appearance can help distinguish this from other rots.
Rhizoctonia - Cuttings rot and infected roots decay rapidly. Reddish-brown lesions may appear just below the soil line. Above ground symptoms are similar to the others with stunting, chlorosis, wilting, defoliation, and death.
Cultural control
- Use healthy planting stock.
- There are differences in cultivar reaction to Fusarium so select ones that are more resistant.
- Plant only in well-aerated, well-drained, pathogen-free media. Also disinfect any tools and equipment that might be used with and contaminate the media.
- Avoid reusing pots or trays from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
- Destroy symptomatic plants.
- Fertilize with nitrate rather than ammonium forms of nitrogen. Monitor EC levels closely so they do not get too high and do not let the media dry out completely. Ideal pH range is 5.4 to 5.8 for garden mums.
- Remove plant debris during and after production.
Chemical control Chemical treatment should be preceded by an accurate diagnosis since most of the chemicals are not effective against all pathogens that might cause these root rots.
For Fusarium: Use in conjunction with other control tactics such as thorough sanitation.
- Heritage at 0.2 to 1 oz/100 gal water then drench apply 1 to 2 pints solution/sq ft. Group 11 fungicide. 4-hr reentry.
- Medallion WDG at 1 to 2 oz/100 gal water as a drench. Using with oils or adjuvants may damage plant. Group 12 fungicide. 12-hr reentry.
- Postiva at 10 to 28 fl oz/100 gal water as a drench. Group 3 + 7 fungicide. 12-hr reentry.
- Thiophanate-methyl-based products. Group 1 fungicides. 12-hr reentry.
- Cleary's 3336 EG at 8 to 16 oz/100 gal water as a drench.
- OHP 6672 4.5 F at 7.5 to 20 fl oz/100 gal water.
For Pythium (or Phytophthora): Rotate fungicides that have a different mode of action for resistant management.
- Banol at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
- Banrot 40 WP at 6 to 12 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Heritage at 0.2 to 1 oz/100 gal water then drench apply 1 to 2 pints solution/sq ft. Group 11 fungicide. 4-hr reentry.
- Mefenoxam 2 AQ at 0.49 to 0.98 fl oz/100 gal water as a soil drench at transplanting. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- MetaStar 2E at 0.5 to 2 fl oz/100 gal water at transplanting, and see label for media incorporation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Monterey Garden Phos at 1 teaspoons/8 gal water as a soil drench. Group P7 fungicide. H
- Proplant at 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
- Subdue MAXX at 0.5 to 1 fl oz/100 gal water as a drench. Apply 1 pint of solution per 1 sq ft. Group 4 fungicide. No restrictions on reentry when used as a soil drench.
Biological control Use in conjunction with other control tactics such as thorough sanitation.
- RootShield Plus Granules (Trichoderma harzianum Rifai strain T-22 and T. virens strain G-41) at 1 to 3 lb/ cubic yard soil mix. No restrictions on reentry required when soil incorporated. O
- Bio-Tam 2.0, Tenet WP, or Obtego (Trichoderma asperellum and T. gamsii) at 0.5 to 1.5 lb/cubic yard of substrate. See label for details and other application methods. No restrictions on reentry when soil incorporated. O
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.