This leaf-spot disease is caused by a fungus (Colletotrichum) that is most active in warm, wet weather. The fungus spends the winter in dead plant tissue. It spreads new spores in the spring. Leaves must be wet for an extended period before infection can take place.

This plant disease, also known as Southern blight, is caused by a fungus (Sclerotium rolfsii) that lives in the soil. The fungus is widespread in the South and attacks many types of plants. It spreads by moving water, diseased transplants, infected soil, and contaminated tools, traveling short distances over the soil to attack adjacent plants. The fungus produces tiny ''mustard-seed'' pellets, called sclerotia; these are white when young, maturing to dark red or brown. Sclerotia survive many years in soil, infecting healthy plants when conditions are suitable, especially in hot, humid weather. In the early 1990s, the disease appeared on hostas in the upper Midwest, perhaps coming from southern-grown plants. It survives cold winters.

Aphelenchoides fragariae This disease is caused by a nematode that infests hosta leaves. Nematodes are microscopic, clear worms that usually infest plant roots, but this one infests the foliage of many plants. Nematodes enter leaves through the breathing pores. Inside, they feed on the tissues and may kill them. Leaf veins act as barriers until the leaf is wet; then the nematode swims through the water to infect other leaves. Foliar nematodes reproduce rapidly and are most damaging in wet-summer, warm regions of the country. They live in soil and dead plant tissue for years; some plants harbor them with no symptoms.

This virus infects only hosta plants. This extremely contagious disease was first identified in 1996, but it may have been present before that. In 2003 and 2004 the number of infected plants grew rapidly, and the disease is now very wide-spread. The virus develops for 3 or more years with no symptoms, but meanwhile the plant is contagious to other plants. Because the disease is not yet well-known, it has spread rapidly in the stocks of hosta growers in the United States and Holland-many hostas for sale are infected. Avoid purchasing hosta plants with uneven color, blotchiness, or puckered or twisted leaves, and don''t purchase any hostas from a nursery that has suspect plants in its collection. Some varieties may be more resistant to the disease than others, but they have not yet been identified, so a reliable list is not available.

These pests are mollusks and are related to clams, oysters, and other shellfish. They feed on a wide variety of garden plants. Like other mollusks, snails and slugs need to be moist all the time. For this reason, they avoid direct sun and dry places and hide during the day in damp places, such as under flowerpots or in thick groundcovers. They emerge at night or on cloudy days to feed. The young look like miniature versions of their parents.

Hostas are shade plants and cannot tolerate too much direct sun. Sunlight breaks down chlorophyll, the green pigment in plant leaves. Hostas cannot replace the chlorophyll as fast as it is destroyed, and bleach to a cream or white color. If the leaf remains in direct sunlight, the bleached areas then burn from overheating and die, turning tan or brown. They can tolerate more morning sun than afternoon sun, and tolerate more sun in cool weather than in hot weather.