In the last 3 years, I have seen the decline of several thriving hosta beds. I have also heard complaints from homeowners in the same regard. Horticulture specialists tell us that too much water or too much sun can cause hostas to decline. I am sure that is correct, but the situations reported to me and the few cases I have seen have not had a significant change in moisture or light. So what is the problem? Are they too crowded? Maybe in a few cases that is true, but the dieback should stabilize if that were the case because affected plantings are now very thin. The clinic has seen an increase in hosta problems in recent years, and listed here are a few disease problems that could be involved in the decline. Some have been very severe in isolated cases. Check your plants for symptoms now or mark your calendar to check for these problems in early spring.|
Anthracnose is a fungal leaf disease that causes irregularly shaped white to tan spots on the leaves. Usually a brown border surrounds the affected area. The spots become torn and give the leaves a tattered appearance. If you examine the leaves with a hand lens the day after a rain, you can see little black hairs (named setae) of the fungus sticking out of the less obvious fruiting bodies. The causal fungus is a Colletotrichum species. The disease is common in warm, wet conditions, so if you can maintain good plant spacing you might see less anthracnose. Fungicides are an option if the disease is a chronic problem. If you choose to spray, initiate use of a protective fungicide starting as leaves begin to emerge and repeat throughout the wet season. Fungicides registered for this use include many of the copper fungicides. There may be other products registered. Read labels carefully to be certain the product is cleared for use on hosta to control anthracnose.
A very nasty crown rot of hosta has invaded Illinois and may be on the increase. The disease is called Sclerotium blight and is caused by a fungus called Sclerotium rolfsii cv delphinii (yes, it also occurs on delphinium). Look for sudden collapse of your plants, mushy leaf stems, a white fungal growth in a fan pattern on the lower leaves, and the presence of small, circular, tan, mustard seedlike fungal structures (sclerotia). The disease is very difficult to control. Although it was previously thought that this fungus could not survive Illinois winters, the pathogen has shown otherwise, at least in protected locations and in mild winters. Current research in Iowa is looking into the parameters involved in winter survival of this fungus as well as some specific hosta cultivar reactions to infection by Sclerotium. As this information becomes available next year, we will keep you posted. At present the management of Sclerotium blight focuses on removing infected plants and soil (to 8" depth) from immediately around the plant. The sclerotia remain in the soil for a long time and serve as overwintering structures for future infection. Chemical control is difficult, but PCNB (terrachlor) is registered on some perennials and may be used to stop disease spread. Check the label for crop registration before use. Sclerotium also appears to be able to grow in old bark mulch, so consider removing bark mulch from around the base of hostas or at least replacing it often. Coral bells, lamb's ear, and lady's mantle are plants suggested as suitable replacements of hostas if a less susceptible species is desired.
Foliar nematodes can also wreak havoc on hosta growth. These microscopic round worms cause necrosis between veins or in blocky appearing areas on the leaves. Eventually entire leaves and plants may die. These nematodes are found in the plant but not in the soil. The foliar nematodes are discussed in more detail in issue no. 16 of this newsletter. The nematodes spread from plant to plant in splashing water and may live over winter in protected crowns of perennial plants. There are no chemical controls available for this nematode. Avoid excessively wet foliage and close spacing of plants. Discard contaminated stock (take away in a plastic bag) and inspect new plants carefully. We have not seen foliar nematodes in retail hostas in Illinois, but this is a current concern in the green industry and a problem in many southern and eastern states. (Nancy Pataky)