Flowering Diminished due to Overcrowding
When lilies of the valley become overcrowded, the flowers become sparse. Simply divide or thin the plants, and next year more flowers will appear.
Leaves Yellowed because of Normal Leaf Senescence
In late summer, the leaves of lily-of-the-valley normally turn yellow and begin to look a little sickly. They die back every year. Keeping the bed well watered through the hot summer month’s delays this natural process a bit.
Leaves Notched caused by Weevils
Weevils are beetles that have a very distinctive long snout. Most are between 1/8 and ½-inch long. They have hard shells and are usually brown or black. They are active at night and hide in soil and debris during the day. Weevils sometimes chew lily-of-the-valley leaves along the edges, notching them. They usually don't attack these plants until well after flowering time. Even then, they do not seriously harm them. Spray foliage of affected plants with neem insecticide, taking care to cover both sides of the leaves, two or three times at 7 to 10 day intervals to kill weevils that are feeding on the leaves. For long-term control, spray beneficial predatory nematodes to the soil around the plants to attack the weevil larvae living there. For more information see the file on Controlling Weevils
Ragged Holes In Leaves means Slugs and Snails.
Slugs are essentially snails without protective shells. They are 1 to 2 inches long (some species grow up to 8 inches). They may be white, gray, yellow, or brown-black. Slugs and snails are attracted to moist, well-mulched gardens and acidic soil, just the type of environment lilies-of-the-valley like. Slugs and snails are active at night, rasping ragged holes with their file-like tongues in leaf and stem surfaces. They hide under rocks, boards or leaf litter during the day. These pests are always most destructive in shaded gardens and during rainy spells. Trap them in commercial traps or shallow plates baited with beer. Attracted to the yeast in the beer, they climb in and drown. Begin trapping within the first three to four weeks after the last frost. For more information see the file on Controlling Slugs
Leaves and Stems Die Back; Roots Rot due to Crown Rot
A soil-dwelling fungus that occasionally attacks lilies-of-the-valley at the soil line causes crown rot. The leaves are discolored as they emerge, and the young shoots wilt. Roots blacken, rot, and become covered with white fungal threads. The whole plant dies in a few days. There is no cure for this disease. Remove and discard the infected plants and the surrounding soil. Do not replant lilies-of-the-valley in this spot for a year or two. For more information see the file on Controlling Fungal Disease
Brown Spots on Leaves indicates Fungal Leaf Spot.
Sometimes lilies-of-the-valley develops leaf spots. Caused by various fungi, these circular brown spots are very different from the normal browning of the leaves toward the end of the season. Pick off and discard affected leaves. In severe cases, spray plants with a sulfur-based fungicide every 7 to 10 days until symptoms begin to disappear. Remove dead plant debris promptly from the garden to reduce over wintering spore populations. Destroy seriously infected plants, together with the surrounding soil in the trash to avoid spreading the fungus. For more information see the file on Controlling Fungal Disease
Leaves Spotted; Stem Bases Rotted means Stem Rot.
A fungal stem rot infects lilies-of-the-valley growing in very humid conditions. At first, the leaves develop yellowish or grayish specks, which later turn into dark brown sunken spots. The disease then spreads downward into the lower parts of the plant to the crown, which rots. Remove and discard infected plants, or cut away affected plant parts with a clean, sharp knife or razor blade. Disinfect tools after use in a solution of hot water and household bleach. To reduce moisture around plants, lighten heavy soil by adding perilite, vermiculite, or peat moss to provide good drainage. Avoid over watering. Space plants farther apart to prevent crowding. For more information see the file on Controlling Fungal Disease