Plant Diseases

A plant disease is any harmful condition that affects a plant’s appearance or function. Common pathogens that cause diseases include: fungi, bacteria, and viruses. Some nematodes are plant disease agents. Temperature extremes or nutrient deficiencies are examples of disorders caused by non-infectious factors.

The disease triangle is a fundamental concept in plant pathology. Disease occurs only when all three sides of the triangle are present: a susceptible host, a pathogen (the agent that causes disease), and an environment favorable for disease to develop. Plant diseases are managed by manipulating the disease triangle: the plant, the pathogen, and/or the environment.

Infection begins when the pathogen enters the plant. The disease process starts when it arrives at a part of a plant where infection can occur. If environmental conditions are favorable, the pathogen begins to develop. The plant is diseased when it responds.

Plants respond to disease in 3 main ways:

1. overdevelopment of tissue - galls, swellings, or leaf curls;
2. underdevelopment of tissue - stunting, lack of chlorophyll, or incomplete development of organs; or
3. tissue death - blight, leaf spot, wilting, and cankers.

Plant disease pathogens may be spread in many ways:

• by wind;
• rain;
• animals;
• soil;
• nursery grafts;
• vegetative propagation;
• contaminated equipment and tools;
• infected seed stock;
• pollen;
• dust storms;
• irrigation water; and
• people.

Infectious Organisms that Cause Diseases

Fungi are the most common infectious organisms causing plant disease. They do not have chlorophyll so they cannot make their own food. They must get it by living on another organism.

Most fungi are beneficial. They contribute to soil fertility by releasing nutrients from dead plants or animals. Those that cause plant diseases are parasites on living plants. They may attack plants and plant products both above and below ground. Some fungal pathogens attack many plant species; others have to only one host species.

Most fungi reproduce by spores, which function like seeds. Fungus spores are often microscopic and are produced in tremendous numbers. Often spores can survive for weeks, months, or even years without a host plant. Excessive water or high humidity is nearly always needed for spore germination and active fungal growth.

Spores can spread from plant to plant and crop to crop through wind, rain, irrigation water, insects, and by people through infected clothing and equipment.

Fungal infections frequently are identified by the vegetative body of the fungus (mycelium) and the fruiting bodies that produce the spores. Often, they are large enough to see.

Symptoms of fungal infections include

• soft rot of fruits,
• plant stunting,
• smuts,
• rusts,
• leaf spots,
• wilting, or
• thickening and curling of leaves.

Powdery and downy mildew, smut, root and stem rots, and sooty and slime molds are examples of fungus diseases.

Bacteria are single-celled organisms that usually reproduce by simple cell division. Some divide as often as every 30 minutes. They can build up quickly under warm, humid weather conditions. Leaf, growing shoots, and fruit diseases are the most common types in Kentucky.

Bacteria can be carried from plant to plant in water droplets, by wind, rain splash, insects, or on equipment. They often survive between growing seasons in crop residue, in seeds or cuttings, or in weeds.

Leaf Diseases Caused By Fungi and Bacteria

Leaf Spots

Bacteria or fungi can cause leaf spots that vary in size, shape, and color. Usually the spot has a distinct margin and may be surrounded by a yellow halo. A fungal leaf spot nearly always has a growth of some type in the spot, particularly in damp weather. It may be a tiny pimple-like structure or a moldy growth of spores. Often the structures are visible through a hand lens. Nearby diseased areas may join to form irregular "blotches."

Leaf Blights

Leaf blights generally affect larger leaf areas and are more irregular than leaf spots.

Rusts

Rust fungi often produce bright yellow, orange-red, reddish-brown or black "pustules". The pustules are usually raised above the leaf surface. Rust can be rubbed off the leaf surface.

Powdery Mildews

Powdery mildews are diseases caused by some closely related fungi. The mildew can be diagnosed by a grayish white powder mat leaves and in some cases stems. Affected leaves usually turn yellow, wither and die rapidly.

Downy Mildews

Wilts, Root and Crown Rots

The main symptoms of these three diseases are wilting, stunting, and death. The causal organisms are usually soil-borne, that is they are already present in the soil when the host crop is planted. Most other diseases are usually spread through the air. Some of these diseases may be controlled through the use of soil fungicides and/or soil fumigants but most are controlled with resistant varieties and cultural practices. Good controls are not available for many of these diseases.

Wilts

Most wilt diseases are caused by fungi -- Fusarium and Verticillium -- and a bacterium, Erwinia. Each parasite causes wilts on a wide range of crop plants. A light to dark brown streaking often can be seen when the stem of a diseased plant is cut lengthwise. However, it is often difficult to determine which of these wilt diseases a plant may have.

Plants with Fusarium or Verticillium wilt grow slower than healthy plants and may show stunting effects before wilting occurs. The lower leaves usually turn yellow and wilt first, then yellowing and wilting slowly progress up the plant. It may take several days to a few weeks from first wilting and plant death.

Crown Rots

 

Root Rots

A great variety of fungi can infect the crown of the alfalfa plant, which is the portion of the taproot just below the soil line. The alfalfa plant stores food reserves for winter in the crown. Crown rot diseases interfere with this process, making plants more susceptible to winter injury.

Seed and Seedling Diseases

Seed and seedling diseases occur on plants from after germination until about 1 or 2 weeks after seedling emergence. Fungi that are on the seed at planting or are present in the soil are the cause. They often are responsible for poor emergence and thin stands.

Seed and seedling diseases are most common in cool, wet soils. They are controlled by planting crops in warm soil, by fungicide seed treatments and by use of in-furrow, broadcast, or band-applied fungicides.

Smuts

 

Viruses

Viruses are too small to be seen with a microscope. They are generally recognized by their effects on plants. There can be a variety of responses: stunted growth; change in plant color; abnormal formation of infected roots, stems, leaves, or fruit. Mosaic diseases, characterized by light and dark blotchy patterns, usually are caused by viruses.

It can be difficult to distinguish between diseases caused by viruses and those caused by fungi and bacteria. A positive diagnosis requires sophisticated testing, such as inoculating indicator plants and observing the results or using specifically identified antibodies to test for the presence of the organism.

Viruses depend upon living organisms for food and reproduction; they cannot exist very long outside a host. Viruses are commonly spread from plant to plant by mites, aphids, leafhoppers, or whiteflies. A few are spread in the seeds of the infected plant.

 

 

Nematodes

Nematodes are small, usually microscopic roundworms with mouthparts like hollow needles – called stylets. Stylets are used to puncture and feed on the contents of plant cells. Nematodes may develop and feed inside or outside of a plant. They are easy spread on footwear, tools, and equipment.

The life cycle of a nematode includes an egg, several larval stages, and an adult. Most larvae look like small adults. In unfavorable conditions, females of some species form inactive, resistant forms called cysts. The cyst is the hard, leathery, egg-filled body of the dead female, which is difficult to penetrate with pesticides. A cyst may protect eggs for as long as 10 years.

Typical aboveground symptoms are stunting, yellowing, loss of vigor, general decline, and eventual death of plants. Since many other problems can cause symptoms root and soil samples need to be examined when nematode injury is suspected. Nematodes are controlled with cultural practices such as crop rotation, resistant varieties and nematicides.

Diagnosis of Plant Diseases

A correct diagnosis is the first step in disease management. To recognize a disease condition, you must know the plant's normal growth habits. When you are trying to identify the cause of a plant disease, you need to look for symptoms - the host plant's reaction to the disease agent, and signs - visible presence of the disease agent.

Many different plant diseases cause similar symptoms. Different pathogens and agents that are not pathogens can cause leaf spots, wilts, root galls, or stunted growth. For example, similar symptoms may be a result of mechanical injury, improperly applied fertilizers and pesticides, or frost. Often, the only way to pinpoint the cause is to find the observable signs that the particular disease agent is present -such as fungal spores and mycelium or bacterial ooze.

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