Gray mold on Plants

Symptoms

Look first for tiny, almost translucent spots on leaves or petals of your plant. Those spots turn brown and appear water-soaked. You may also see brown, wet spots on leaves or flowers, and the flower petals and leaves can look spotted.

Infected buds fail to open. Affected tissue decays, discolors, and wilts, and infected terminals die back. On green canes, large, diffuse, target-like lesions form. These lesions are large and diffuse, unlike the tiny spots found on leaves or petals.

On decaying tissue, grayish fungal spores develop and give the area a fuzzy or woolly appearance. On infected tissue you may also see dusty gray spores. Declining or dying soft plant tissue is one place where infection can take hold.

Petals, leaves, buds, terminals and canes can all show signs of gray mold. Rose, hydrangea, azalea, fuchsia and strawberry are among the plants it is recorded on. Cacti, impatiens, oleander and tree peony are listed as well. Bird of paradise and giant sequoia are on the list too.

Causes

Gray mold is particularly troublesome under high humidity and moderate temperatures of 70 to 77°F (21 to 25°C). Relative humidity above about 90% for 6 or more hours is one listed condition. Continuously wet conditions for 6 or more consecutive hours are another.

Frequent wetting of foliage by overhead irrigation is also a cause. Crowded and shaded conditions with poor air circulation are listed as well. Treatment advice names drip or microsprinkler irrigation as an alternative to overhead watering.

Weeds and plant debris can harbor spores. Declining or dying soft plant tissue can become infected. Old blossoms, declining green plant tissue, fallen leaves and weeds are all named as spore sources in the treatment advice.

Several of these causes are about water and air around the plant. Others are about what is left on and near it, such as debris, weeds and dying tissue. Humidity, wetness, crowding and debris each appear as separate entries in the list of causes.

Diagnosis

Begin with the surface of the damaged tissue, where grayish fungal spores give a fuzzy or woolly appearance on decaying parts. Dusty gray spores on infected tissue point to gray mold. Spots on leaves or petals are part of the picture, along with the gray spores. What sets gray mold apart from a general care problem is the fungal growth: grayish spores on decaying tissue, not just spotting or wilting.

Next, compare the conditions around the plant with the listed causes. Overhead watering that wets the foliage, crowded and shaded plantings with poor air circulation, and humidity above about 90% for 6 or more hours all fit gray mold.

Then check which parts are affected. Buds that fail to open, terminals that die back, and large, diffuse, target-like lesions on green canes are all listed symptoms. Affected tissue that decays, discolors and wilts is listed as well.

Last, look at the kind of plant you have. Gray mold is recorded on azalea, rose, hydrangea, strawberry and the other plants named on this page. This site does not identify plants or diagnose from photos, so the comparison is yours to make using the signs on this page.

Treatments

Start with cultural steps. Remove and dispose of old blossoms, declining green plant tissue, plant debris, fallen leaves, and weeds to reduce spore sources. Weeds and plant debris harbor spores, and removing them is one way to reduce spore sources. Prune and discard infected plant parts as soon as a problem is detected. The timing in the advice is as soon as a problem is detected. Discard severely affected plants, and keep tools and pots clean. Pots are named as well as tools, so clean containers as well.

Avoid wetting foliage, such as by using drip or microsprinkler irrigation instead of overhead watering. Overhead irrigation wets foliage frequently, and drip or microsprinkler irrigation is the alternative named. Improve air circulation around plants and avoid crowded, shaded plantings. Give plants proper cultural care to keep them vigorous, and use good sanitation practices to modify environmental conditions where feasible.

Chemical options come last. The fungicide active ingredients listed are chlorothalonil, thiophanate methyl and triforine. Apply a fungicide preventatively, before disease develops. Fungicides act only as a preventative and generally are not very effective in landscapes once conditions favor gray mold development.

The product label is the legal instruction, and you must follow it for the crop and site being treated. This page gives no rates, dilutions or spray intervals.

Prevention

Prevention is about conditions. Frequent wetting of foliage by overhead irrigation is a listed cause, so keeping leaves from being wet often matters. Drip or microsprinkler irrigation avoids wetting foliage. Avoid crowded, shaded plantings, and improve the air circulation around them.

Clear away old blossoms, fallen leaves, plant debris and weeds, which can harbor spores. Remove declining green plant tissue, since dying soft tissue can become infected. Keep plants vigorous with proper cultural care and good sanitation. Clean tools and pots as part of that routine.

Moderate temperatures between 70 and 77°F (21 to 25°C) combined with high humidity are the pairing to watch. Humidity above about 90% for 6 or more hours, and wet conditions lasting 6 or more consecutive hours, are the thresholds the causes list.

Plants most often affected

Landscape hosts for gray mold include azalea, bird of paradise, cacti, coast redwood, fuchsia, giant sequoia, hydrangea, rhododendron and rose. Strawberry is also affected, and impatiens, oleander and tree peony are on the list as well. If your plant appears here, the signs and conditions on this page are worth comparing with what you see.

• Chinese Banyan
• Cultivated Garlic
• Garden Cosmos
• Madagascar Periwinkle
• Poinsettia