How to Treat Diseased Agapanthus

You may not expect water to bother Agapanthus plants (Agapanthus spp.) because they're commonly known as lilies-of-the-Nile, but prolonged wet conditions expose them to several fungal diseases. One of those diseases transforms their globes of flowers into balls of fuzz. Others attack their arching green leaves or rot their roots or bulbs. Affecting both in-ground and container plants, these diseases occur where lily-of-the-Nile varieties grow outdoors as perennials, U.S. Department of Agriculture plant hardiness zones 6 through 11.

When treating a diseased lily-of-the-Nile, dress to protect yourself from the plant's toxic, skin-irritating sap.

Gray mold fungus preys on dying flowers of lily-of-the-Nile. It spreads most quickly when humidity is high and the air temperature is 70 to 77 degrees Fahrenheit. Wind carries the fungus from previously infected plant debris to healthy plants, where it devours the flowers and germinates into masses of grayish spores.

A combination of cultural measures may prevent gray mold:

Plant lilies-of-the-Nile 2 to 2 1/2 feet apart for good air circulation. 

Remove and dispose of old or damaged flowers and leaves as well as debris that may harbor fungal spores.

Water from beneath the plants. The spores can't germinate without standing water. So keeping the leaves and flowers dry is essential.

Consider spraying the plants with organic, ready-to-use neem oil. To prevent gray mold, one neem oil manufacturer recommends spraying plants every one to two weeks during warm, humid or wet weather. Spray the plants until the neem oil runs from all the flowers and leaves.

Treat existing gray mold weekly with ready-to-use neem oil until the symptoms subside. Then spray the neem oil every two weeks to prevent reinfection. Spray the affected plants until the neem oil runs from their flowers and leaves.

Remove infected plant parts with clean, sharp stem cutters disinfected in rubbing alcohol before each cut and after you finish to prevent the spores from spreading.

Dress in protective clothing and gear -- including a long-sleeved shirt, long pants, closed-toe shoes, chemicalproof gloves, safety goggles and a respiratory mask -- before spraying neem oil.

Neem oil is toxic to bees. Spray it in early morning or after dusk, which is when bees aren't active.

Water splashes anthracnose leaf-spot fungi onto lily-of-the-Nile leaves. Anthracnose fungi cause yellow areas -- sometimes with tan or rust-colored spotting -- along leaf margins; the infection moves inward, and the leaves eventually die.

Prevent or treat anthracnose leaf spot as you would gray mold, with adequate spacing, diseased leaf and debris disposal, and proper watering. Start spraying the plants with neem oil as soon as their leaves emerge in spring, completely covering the entire plants. Spray again weekly while rain is likely.

Bulb- and root-rot fungi thrive in warm, wet soil.

Bulb rot invades damaged bulbs, causing yellowing, dying leaves. In advanced stages of the disease, entire leaf clumps collapse. The rotted bulbs are filled with pinkish-brown decay and pull from the soil without resistance. The disease remains infectious even in bulbs lifted out of the ground for storage.

Like bulb rot, root rot yellows and wilts the leaves. It may stunt or even kill the plants. Affected roots are dark , decayed and fragile.

Damage control takes precedence in treating bulb and root rot. Dispose of the diseased plants and infected soil. Discard potted plants along with their contaminated growing medium and containers. Dealing with infected in-ground lily-of-the-Nile plants, however, takes more effort:

Long pants 

Long-sleeved shirt

Closed-toe shoes

Waterproof gloves

Pruning shears

Rubbing alcohol

Sealable plastic bags

Perennial spade with sharp blade

Rake

Heavy cardboard

Compost, wood chips or other organic mulch

Ruler or tape measure

Dress in long pants, a long-sleeved shirt, closed-toe shoes and waterproof gloves.

Prune each bulb rot-infected or root rot-infected lily-of-the-Nile clump's leaves back to the soil with pruning shears. Disinfect the pruning shears in rubbing alcohol between cuts and after pruning.

Put the removed leaves in sealable plastic bags.

Dig around each lily-of-the-Nile clump, working a perennial spade's sharp blade down and under the center of every clump to loosen the roots.

Lift the loosened clumps out of the ground with as much of their soil as possible.

Check the area where the plants were for root remnants. Seal the root remnants and the loosened clumps in the plastic bags with the removed leaves for disposal.

Rake the soil smooth. Cover it with heavy cardboard.

Cover the heavy cardboard with a 3- to 4-inch layer of organic mulch, such as compost or wood chips. The mulch will prevent sunlight from reaching any root remnants, which will eventually die.

Disinfect the spade and rake with rubbing alcohol before reusing them.

Seal discarded container plants in plastic bags.

The cardboard and mulch eventually break down, improving the soil's structure and nutrients.

To prevent future problems, plant undamaged, certified disease-free lily-of-the-Nile bulbs in well-drained soil, and water their soil only enough to keep the plants healthy.