Plant disease is an impairment of the normal state of a plant that interrupts or modifies its vital functions. Plant diseases are caused by biotic (living) pathogens like bacteria, fungi, nematodes, viruses, and phytoplasmas, as well as abiotic (nonliving) problems.

Abiotic problems include adverse extremes in the environment, such as nutrient deficiency, prolonged water stress, air pollution, insufficient or too much light, too much herbicide use, automobiles, weed whackers, wind, lightening, dog urine, and almost anything else in the environment. These can be managed with cultural practices such as limiting herbicide use, keeping dogs away from sensitive areas, protecting tree trunks from weed whackers and lawn mowers and planting the right plant in the right place.

Biotic factors include living cells such as bacteria, fungi, nematodes, phytoplamas and viruses. Bacteria are single-celled organisms with no nucleus. Most bacteria associated with plants are saprophytic (feed on dead organic debris) and do no harm to plants. Bacteria can cause blights, leaf spots, fruit rots, vascular wilts, and galls. They typically enter the plant through natural openings, such as hydathodes and lenticels, or wounds created by wind-swept sand, hail, heavy rain, and/or mechanical damage. They can be spread by infected seed or from plant to plant by water splashing, insects, and humans.

Fungi usually grow as threads or strands called hyphae. A single hypha can be only a few inches long or miles long. Fungi grow on dead or rotting plants. Mushrooms are one type of fruiting body that some fungi form to reproduce. Fungi reproduce via the production of spores. These spores may be spread long distances by air or water, or they may be soilborne. Certain fungi are also able to produce structures that enable them to survive in the soil for many years. They can cause root and stem rots, shoot and leaf blights, leaf spots, cankers, vascular wilts, and postharvest storage rots.

Nematodes are tiny microscopic round worms that generally live most of their lives in the soil. Plantpathogenic nematodes comprise only a very small subset of all the nematode species. These nematodes have a needlelike mouthpart called a stylet used to pierce the plant cells and feed on the cell contents. Feeding either kills the plant cells, leading to the development of lesions, or causes galls to develop on the roots, reducing the ability of the plant to take up water and nutrients.

Phytoplasmas are specialized bacteria without a cell wall that depend on the host plant to survive. Typically transmitted by insects during feeding, they live in the plant sap and clog up the vascular system, reducing the ability of the plant to move water or nutrients. A typical symptom is phyllody, the production of leaflike structures instead of flowers, or a witch’s broom or bushy appearance.

Viruses are pieces of RNA or DNA with a protein coat. They multiply by inducing host cells to form more virus particles. Viruses are spread through vegetative propagation (cuttings, grafting, etc.), seed, insects (most common), or nematodes. Typical symptoms include stunting, mosaic, or ring spot patterns on leaves and fruit.

Plant diseases are identified by their signs (visual presence of the pathogen) and symptoms (the visual effect of the pathogen on the plant).

So how to identify what is wrong and then treat it effectively?

First, identify the plant. Some plants are more prone to some diseases than other plants. Knowing the plant is a big step in limiting the potential causes.

Second, look around to see if other plants in the area are suffering. Notice the distribution of the problem. Is it limited to one part of the plant or spread throughout? Is it only on one side? Where is it relative to cars, dogs, sprinklers etc? If so, there may be an abiotic cause (drought, temperature, pollution etc.?)

Third, are there specific symptoms or signs? Signs include mushrooms, slime or other structures adjacent to the plant. Symptoms on the plant may include spots, blights, blisters, root discoloration, rot, leaf striping, galls, yellowing, mosaics, ring spots, or mottling.

Fourth, determine the cause. Consider the soil, cultural practices including irrigation, fertilization, mowing and use of pesticides. If still not sure, ask for help. the Horticulture Answer Line at the Botanical Garden is staffed by Master Gardeners ready to help. Only then, will treatment of the identified problem be possible!

Diseases Common to This Area and Their Management

Powdery Mildew

Most powdery mildew are very host specific, mildew on cucumbers will not infect roses. Look for white, powdery growth on leaves and shoots. Warm days and cool nights are ideal for powdery mildew development. Particularly susceptible: roses, maples, gooseberries, phlox, nine bark, hascaps, lupines, lilacs, sage, squash and cucumbers. To treat it avoid planting in shady areas or areas with poor air circulation. Fertilize properly because too much nitrogen causes lush foliage and shade, providing conditions for fungal growth. Plant powdery mildew resistent cultivars. Control mild to moderate infections with horticultural oil or with plant-based oils, such as neem oil. If really necessary, a fungicide can be applied.

Black Spot.

This fungal disease of roses is usually a problem anywhere leaves commonly remain wet, such as where roses are sprinkler irrigated during the evening or night. The fungus produces black spots on the upper surface of rose leaves and stems. Spots sometimes have feathery margins, and tiny, black, fungal fruiting bodies may be visible in the spots. Yellowed areas develop around spots. Infected foliage often drops prematurely. The pathogen persists on living and dead leaves and on infected stems. Its spores are spread by splashing water. For spore production and infection, leaves must remain wet for more than about 7 hours. To reduce black spot, irrigate and hose off aphids in the morning instead of the evening or night. Do not plant roses too close together. Prune canopies to increase air circulation. Prune off infected stems during the dormant season and dispose of fallen rose leaves and stems away from rose plants. Avoid planting roses where they will not receive at least 6 to 8 hours of full sun. Choose disease-resistant varieties when planting. Miniature roses are more susceptible than other types, but a few varieties are reliably resistant to all strains of black spot.Where weather favors severe disease, available preventive fungicides include neem oil.

Bacterial Canker or Blight

Blight spreads by fungal spores that are carried by insects, wind, water and animals from infected plants, and then deposited on soil.¹ The disease requires moisture to progress, so when dew or rain comes in contact with fungal spores in the soil, they reproduce. When it rains, water hits the ground, splashing soil and spores onto the lower leaves of plants, where the disease shows its earliest symptoms.

Blight spreads by fungal spores that are carried by insects, wind, water and animals from infected plants, and then deposited on soil. The disease requires moisture to progress, so when dew or rain comes in contact with fungal spores in the soil, they reproduce. When it rains, water hits the ground, splashing soil and spores onto the lower leaves of plants, where the disease shows its earliest symptoms.

While there is no cure for blight on plants or in the soil, there are some simple ways to control this disease. 

Cultural practices are important to prevent blight. When planting tomatoes, make a plan to prevent blight. Incorporate these simple steps to keep your plants healthy.

1. Practice crop rotation by planting tomatoes in a section of the garden that has not been used to grow tomatoes or any other member of the Solanaceae family, such as eggplant, potatoes or peppers, in the last two years.⁵
2. Read seed packages or plant labels carefully to select a tomato variety that is resistant to blight.⁶
3. Stake or cage tomato plants so that foliage grows vertically, off the ground.⁴
4. Mulch well around plants.
5. When watering, use a soaker hose rather than an overhead sprinkler. This will reduce the amount of water on leaves and keep spores in the soil from splashing on plants.

Once blight is positively identified, act quickly to prevent it from spreading. Remove all affected leaves and burn them or place them in the garbage. Mulch around the base of the plant with straw, wood chips or other natural mulch to prevent fungal spores in the soil from splashing on the plant. A fungicide may be necessary but consider this carefully if using on food crops such as tomatoes.

Rust

Rusts are fungi are named for the dry to gelatinous, orange, reddish, or yellowish fruiting bodies and spore masses that many species form on infected tissue. Rusts infect many hosts, including birch, cedar, cottonwood, cypress, fuchsia, hawthorn, juniper, pear, pine, poplar, rhododendron, and rose. Rust fungi and their damage usually occur on fruit, green stems, and leaves of broadleaf plants or on the bark or needles of conifers. Rusts infect hosts when plant surfaces are wet and temperatures are mild, mostly during the fall to spring.

Infected foliage on broadleaf hosts may become spotted, turn yellow or brown overall, and drop prematurely. Infected conifers may develop bushy growth (e.g., witches’ brooms), cankered, galled, and oozing bark and limbs, and spotted needles.

Many rusts have minimal affect on plant health, but certain rusts can kill their hosts. The importance of rust damage varies greatly, such as depending on local environmental conditions and the abundance and nearness of other hosts that are sources of infectious spores.

Cultural practices include avoiding overhead watering. Collect fallen, infected leaves and needles and dispose of them away from host plants. Cut off and dispose of diseased shoots and branches as soon as they appear, except do not prune woody parts so extensively that plants are seriously damaged. Removing nearby alternate hosts of the fungus may help to reduce new infections in certain situations. Plant rust-resistant cultivars if available. If absolutely necessary a fungicide may be used but requires grequent applications to be effective.