How to Identify Common Plant Diseases

Learn how to spot common plant diseases and what you can do about them.

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Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Photo By: Julie Martens Forney

Gray Mold

Mold in the garden is a common sight, especially during extended bouts of rainy, cool weather or high humidity. High moisture levels help mold to grow and spread. In botanical circles, mold that forms on dying flowers or leaves is known as botrytis or gray mold. Botrytis spores are present in soil and frequently already on plants that you buy. It lies dormant until conditions are right for it to blossom and spread. The best prevention is to remove dead or dying leaves, flowers and fruit, and to mulch soil so that rain can’t splash spores onto plants. Flowers that often get botrytis include zinnia, peony, geranium — plants with blossoms that are full and packed with petals.

Leaf Spots

Warm air temperature and days of rain can lead to leaf spot diseases on many different plants, including flowers, vegetables, trees, shrubs and herbs, like this golden oregano. Leaf spots originate with bacteria or fungus, both of which reside in soil or on nearby plants. The combination of warm air and rain foster ideal conditions for an outbreak, especially when leaves stay wet 24/7. Leaves develop individual spots, which eventually coalesce and kill the leaf. Once a leaf falls, filled with infection, it slowly rots and infects the rest of the plant. Gather and destroy any leaves that fall as a result of leaf spot diseases. When rain stops and plants dry out, they will usually outgrow the symptoms. This golden oregano looked beautiful about two weeks after this outbreak and stayed healthy the rest of the season. To prevent leaf spot, make sure mulch covers soil beneath plants to keep disease from splashing onto leaves.

Powdery Mildew Early Infection

Powdery mildew is caused by a fungus that multiplies when soil is dry and humidity is high. It often appears on plants in late summer. The first easily visible signs on leaves are white spots of fungus. Over time, the fungus grows and forms a network on the leaf, connecting the spots. After the fungus overtakes a leaf, the infected parts turn brown and disintegrate. Plants typically attacked by powdery mildew include bee balm (shown), zinnia, roses, lilac, squash, beans, cucumbers and tomatoes.

Powdery Mildew Advanced Infection

This is what powdery mildew looks like when a fungus-ridden leaf starts to fall apart. There are many different types of powdery mildew; they’re actually rather specific to the plants they infect. The fungus that attacks your squash won’t, for instance, spread to your lilac. Treatments include disposing of infected plant material and cleaning up the garden well in fall to remove any infected leaf or stem residue. Destroy infected plant parts; do not add to your compost pile. Mulching beneath plants helps preserve soil moisture and prevent fungus from splashing from soil to leaves. It also helps if plants have good airflow around them — space them properly.

Peach Scab Disease

A fungus is the culprit behind the black spots on peaches. It’s known as peach scab, and round spots that start out small and green slowly become black and almost velvety. With heavy infections, peaches may crack or be misshapen. As with most fungal diseases, wet weather is the trigger for infection to occur. It’s okay to eat peaches with scab. Just peel the fruit and remove any soft or brown spots. To help prevent this disease, clean up all fallen leaves, twigs and fruit. Prune to open up the tree’s inner canopy and increase air flow. Check with your local extension office for fungicide spray recommendations, which should start when petals fall from flowers in spring. Peach scab also affects nectarines and apricots.

Bud Blast

Nothing is more disappointing than watching peony buds develop and then fail to open. This condition is known as bud blast, and while it sometimes involves growing conditions, it’s often due to fungus. In bud blast, buds may develop to marble size and stay hard and dark, never enlarging but rather shriveling. Or they may start to open and reveal dried petals that are brown and hard. Often bud blast occurs with new peonies grown from divisions — and the condition disappears as plants mature. Other times wet spring weather leads to disease development. When peonies get bud blast, remove affected blooms and destroy them. Cut down leaves in fall and destroy those, too. Remember to sterilize pruners between cuts by wiping them with rubbing alcohol.

Early Blight on Tomato

A fungus (Alternaria solani) that occurs naturally in soils causes early blight on tomatoes. The name is a bit misleading because it can attack plants at any point in the growing season. The conditions that favor this disease are high temperatures, rain and high humidity. Rain can actually spread the disease from soil to plants — a raindrop splashes when it hits soil, and that droplet carries fungus to your prize tomato or potato. This leaf shows the classic symptoms of early blight on a tomato leaf: dark spots with a yellow halo. The dark spots eventually dry up and fall out of the leaf, creating holes. To help prevent early blight, mulch soil beneath tomatoes and remove and destroy any infected leaves, sterilizing pruners between cuts. Spraying plants with fungicides can help reduce outbreaks, as can removing all leaves beneath the first fruit cluster.

Late Blight on Tomato

Late blight is another dreaded tomato disease that absolutely destroys a plant. When infection is severe, this fungus attacks leaves, stems and tomatoes. Phytophthora infestans is the fungus behind late blight, and it usually arrives to your 'mater patch on the wind. Wet, humid conditions and high temperatures lead to an outbreak. Keep a watchful eye on plants, and you might spot its first appearance on lower leaves, which you can remove. Late blight doesn’t survive winter on plant debris, so you’re safe composting infected plant parts. Just be sure to bury them deep in the pile to keep from re-infecting your garden while tomatoes are still growing. Fungicide sprays can help keep late blight at bay. It’s also helpful to remove any volunteer tomato seedlings and water early in the day so leaves dry before dark.

Blossom End Rot

Blossom end rot affects many veggies — summer squash, tomato, eggplant, pepper and cucumber. Most gardeners mistake it for a disease, but it’s not. It actually develops due to growing conditions: uneven watering and/or lack of calcium. But blossom end rot often opens the door to secondary diseases, like mold, which can quickly destroy the vegetable. The easiest way to prevent blossom end rot is to add calcium to soil at planting. Use bone meal, gypsum or oyster shells in planting holes. Also make sure plants receive consistent water throughout the growing season, and apply a thick layer of mulch to keep soil moist. Avoid high nitrogen fertilizers, which cause fast leaf and stem growth and inhibit calcium uptake by roots.

Rust

Rust is a fungal disease that forms rusty spots on leaves or stems. There are more than 5,000 types of rust, but common rust attacks many different plants, including hollyhock (shown), roses, daylilies, snapdragons, lawn grass and tomatoes. It appears first on the bottom of lower leaves as white spots. Over time, the spots turn reddish-orange, and eventually black. Rust blossoms when a period of low light (4-8 hours), warm air and moisture (dew, rain or humidity) are followed by bright sunlight for 8-16 hours, coupled with high temperatures and high humidity. That combination of conditions keeps leaves wet, which allows rust to grow. Rust survives winter on infected plants, so it’s vital to clean up and destroy all infected plant parts to help control the disease. Applying fungicides and copper or sulfur sprays can help prevent and slow rust’s attack.

Sunburn on Hosta

Pale, papery spots on hosta leaves that ultimately turn brown and crispy aren’t a true disease, but simply a sunburn. The problem with sunburned hosta leaves is that they weaken the plant as it loses leaves, and diseases can easily take hold. A hosta doesn’t need to be in full sun for sunburn to develop. Even hostas in part shade can suffer if clouds take a vacation. When that happens, water more, which can help plants cope with the sun exposure. If your hosta bed is in a part-shade location, fill it with sun-tolerant varieties, like Hosta plantaginea (the one with the fragrant flowers), 'Sun Power,’ 'Sundance,’ 'Honeybells’ or 'Sun Glow.’

Verticillium Wilt

These beautiful trees are susceptible to a fungus known as Verticillium dahliae. This fungus naturally occurs in some soils and enters a Japanese maple through the roots. Over time, as the fungus multiplies, it blocks water flow in the tree’s internal water transport system. The tree reacts to this invasion by blocking off the infected parts from the rest of the healthy, growing parts. This results in a dead branch in what looks like an otherwise healthy tree. Once verticillium is in the soil, it stays active for 10 years.

Verticillium Ring

If you spot a dead branch on your Japanese maple, cut it and check the end. When the problem is verticillium wilt, you’ll see a dark ring in the wood. Once a tree is infected with verticillium, it’s more sensitive to drought, soil compaction and waterlogged soil. A Japanese maple can recover from verticillium wilt. You can help by first removing infected branches. Next, provide ideal growing conditions — ample water during dry seasons and avoiding mowing or trimming injuries to surface roots or trunk. Create a mulch ring beneath the tree to help protect surface roots. Do not garden beneath Japanese maples, and never use wood chips from a Japanese maple because they could introduce the fungus to your soil. Also, do not use soil from beneath an infected Japanese maple anywhere else in your yard.

Snow Mold

When snow melts in spring, you might spot areas of your lawn that appear to be light tan or even dead. Look closer, and you may see threads of mold — snow mold, to be precise. This turf disease is caused by fungi that grow in cold, moist conditions, like those found beneath melting snow cover. The grass that’s hosting the mold often mats together, which prevents new grass from growing through it. This disease has an easy fix. First, when you spot it in spring, rake grass lightly to break up the mold and lift the matted parts so new growth can resume. Second, prevent the disease in the first place by cutting grass shorter on your last mowing before the snow falls. Tall grass tends to lodge or fall over beneath snow, which leads to matting and snow mold.

Tree Gall

Enlarged bumps that form on tree trunks are known as galls. They’re unsightly but aren’t necessarily a cause for panic. Arborists report that trees with galls usually have a shorter lifespan, but it’s not a reason to remove a tree. Several items cause galls to form. Some kind of injury can create an opening for bacteria, fungus or insects to enter the tree. The tree responds by compartmentalizing the attacker, and a gall forms as a result. While a gall isn’t a death sentence for a tree, it’s a signal that the tree has an uninvited guest inside it. When pruning, be sure to disinfect cutting tools after working with a gall-infected tree to avoid spreading a microorganism to healthy trees.

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