Many of our vegetable and greenhouse producers are beginning to include fall chrysanthemum production in their operations. Mums are usually planted in June and sold in September when fall color is in demand. In Kentucky, mum production can vary in size, and small growers can produce as few as 200 plants per season. Size of production, in turn, can influence cultural practices and initial investment in important practices like surface drainage, pre-plant fungicide dips, and pre-emergent herbicides (Figure 1).
Typically, these plants are set outdoors onto nursery cloth that is in direct contact with the natural ground. Because the most common mum diseases are caused by soilborne pathogens, the threat of disease losses can be as much as 50%, while average losses range from 10% to 25%. In these cases, soilborne pathogens overwinter in soil beneath nursery cloth. If plants are set into the same areas year after year, inoculum builds up and disease risk increases with each passing season.
The Three Most Common Diseases on Mum in Kentucky Are Caused by Soil-borne Pathogens
Pythium Root Rot
Pythium spp. are water mold pathogens (not fungi) that favor cool, wet conditions. Water molds produce swimming spores that move freely in water, increasing risk of infection when water puddles underneath pots. Pythium infects at root tips and then colonizes root systems, causing root loss (Figure 2). In turn, plants wilt from lack of water uptake.
Decaying roots turn black and the root cortex may slough off. Black stem lesions may be visible at soil surfaces. Because Pythium spp. are not true fungi, targeted products should be used for disease management. Products that contain etridiazole or mefenoxam* are most effective. Infected plants are not curable, so preventative disease management is recommended. Cultural practices, including proficient drainage and sanitation, are critical components for a preventative disease management program.
Rhizoctonia Web Blight
The Rhizoctonia fungus does not produce spores, but moves via the growth of threadlike masses called mycelia. Initial infections begin at the soil surface and are responsible for crown rot. Fungal webbing often grows up to upper plant parts when plant canopies become dense and humid (Figure 3).
These web-like mycelia often can be seen without a microscope (Figure 4).
Disease usually becomes a problem as plants mature and foliage does not dry out quickly. Large parts of plant turn brown and necrotic and wilt as the fungus invades branches (Figure 5).
Fungicides containing azoxystrobin, fludioxonil, iprodione, propiconazole, pyraclostrobin, tebuconazole, thiophanate-methyl, trifloxystrobin, and triflumizole* provide effective control. Increase air circulation and promote rapid drying to help reduce disease development. Sanitation is also important to reduce carry-over from one season to the next.
This fungal pathogen invades vascular systems and causes leaf yellowing and plant wilt (Figure 6).
Fusarium fungi infect plant roots and then colonize internal tissue. Collapse of these “water and nutrient highways” can result in starvation of upper plant parts. Often, a single branch or plantlet will show symptoms before the rest of the plant. Necrosis or brown streaks may be visible on outer surfaces of stems, and cross sections usually indicate necrotic (brown decaying) vascular tissue. Often, Fusarium wilt is present with one or more other soilborne diseases. Adjust pH to 6.5 to 7.0 (avoid highly acidic soil). Fusarium wilt is extremely difficult to manage after infection occurs, but fungicides containing azoxystrobin, fludioxonil, and pyraclostrobin* are effective at suppressing the pathogen. Avoid infection by preventing contact with soil or surface water.
By Nicole Ward Gauthier, Extension Plant Pathologist