Rust on Plants

Symptoms

Rust is caused by rust fungi, and its signs are easiest to read on the leaves. Turn a leaf over and check the lower leaf surface. Rust forms dry spore masses or pustules there. Their color ranges across brown, orange, purple, reddish, and yellowish.

Next look at the same leaf from above. The upper leaf surface becomes spotted or turns yellow or brown over the infected pustules. A spot on top that sits directly over a pustule below matches this description.

Infected leaves curl, wither, and drop prematurely. Severely infected plants are stunted. Pustules, spots, curling, and leaf drop all appear on infected leaves, so check more than one leaf before you decide.

Woody plant parts show signs of their own. You may find tissue swellings, galls, or cankers on woody plant parts. Stem dieback is also listed among the symptoms, and death of the entire plant is rare.

Taken together, the listed signs are pustules below, spotting above, curled and dropped leaves, stunting, and on woody parts, swellings, galls, cankers, and dieback.

Causes

Rust is caused by rust fungi. These fungi infect under mild, moist conditions. Most of the other listed causes also involve moisture.

Prolonged leaf wetness is one of those causes. Overhead watering is another, and it favors rust spore germination and spread. The advice given is to avoid overhead watering.

Excess humidity is also listed as a cause. Crowded planting is listed with it. The cultural advice is to prevent excess humidity, provide good air circulation, and avoid crowding plants.

Timing of watering matters too. Water early in the day so that plants dry more quickly, which reduces the length of time foliage stays wet. That speaks to prolonged leaf wetness.

Put together, the causes are rust fungi, mild, moist conditions, prolonged leaf wetness, overhead watering, excess humidity, and crowded planting. Check which of these conditions apply to your plant before you decide what to change.

Diagnosis

Start with the underside of a leaf. Rust is marked by dry spore masses or pustules on the lower leaf surface. Pustules are what to look for, since a spot on the top alone is only part of the description.

Match the top and bottom of the same leaf. If the upper surface is spotted or yellow or brown over infected pustules, that fits rust. Spots with no pustule beneath them do not match the description given for rust.

Next, think about the conditions around the plant. Rust fungi infect under mild, moist conditions. Prolonged leaf wetness, overhead watering, excess humidity, and crowded planting are all listed causes. A plant grown in those conditions that also shows pustules fits the pattern described for rust.

Look at the wider plant too. Rust causes leaves to curl, wither, and drop prematurely, and severely infected plants are stunted. On woody plants, tissue swellings, galls, or cankers and stem dieback are also listed.

Treatments

The listed treatments begin with cultural steps and include one organic option. These steps respond to the causes listed above, which include leaf wetness, humidity, and crowding.

Water early in the day so that plants dry more quickly. Drying more quickly reduces the length of time foliage stays wet. Remove and destroy affected plants or plant parts as soon as they appear.

Prevent excess humidity, provide good air circulation, and do not crowd plants. Choose rust-resistant plant varieties where available to avoid rust problems. Resistant varieties help only where they are available, as the advice says.

The organic option is neem oil. The advice is to apply a fungicide such as neem oil at the first signs of infection. The goal is to prevent serious damage from most rust fungi. Early action runs through the advice, since removal comes as soon as affected parts appear and the fungicide goes on at the first signs.

Follow the product label, which is the legal instruction for the crop and site being treated. Never guess at a dose, rate, dilution, or spray interval. No cited source on this page lists a chemical treatment for rust.

Prevention

Prevention is about the conditions around the plant. Avoid overhead watering, which favors rust spore germination and spread. Water early in the day so plants dry more quickly and foliage stays wet for less time.

Prevent excess humidity and provide good air circulation. Do not crowd plants. Rust fungi infect under mild, moist conditions, so these steps aim at the moisture side of the problem.

Where available, choose rust-resistant plant varieties to avoid rust problems. Prolonged leaf wetness, excess humidity, and crowded planting are all listed causes, so each of these habits removes one of them. Remove and destroy affected plants or plant parts as soon as they appear.

Plants most often affected

Rust is recorded on a number of plants. The hosts listed are aster, carnation, geranium, iris, lily, pansy, primrose, snapdragon, sunflower, sweet pea, juniper, apple, hawthorn, quince and serviceberry. That makes fifteen plants in all. If one of them shows pustules on the lower leaf surface, read the symptoms section to compare.

• Black Baneberry
• Blackeyed Susan
• Blackgum
• Blue Giant Hyssop
• Canada Goldenrod
• Carnation
• Chinese Pear
• Common Buckthorn
• Creeping Jenny
• Eastern Cottonwood
• European Mountain Ash
• European Plum
• French Hydrangea
• Glossy Buckthorn
• Kentucky Bluegrass
• Lawndaisy
• Norway Spruce
• Panicled Hydrangea
• Poinsettia
• Rugosa Rose
• Silktree
• Strawberry Tree
• Sweetscented Joe Pye Weed
• Sweetwilliam
• Weeping Willow
• White Poplar
• Wild Bergamot
• Wild Hydrangea