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One of the major obstacles growers face when raising plants is disease. Plant disease is any harmful condition that makes a plant different in appearance or function from normal. Plant problems can be either biotic or abiotic. Abiotic problems are caused by nonliving stress factors like environmental problems, such as freezes, or cultural, such as overwatering. Biotic problems are those caused by living organisms. In this article we will focus on biotic plant problems. To successfully control plant disease, the first thing you need to do is to correctly identify the cause of the problem so you can use the correct treatment.

Three things are needed for plant disease to occur: the host plant, the plant pathogen, and an environment conducive for the pathogen to infect the host plant. If all three are present, then disease can occur. We can try to manipulate these three to help prevent the occurrence of disease. Plants will respond to disease organisms in several ways. One is by overdevelopment of tissues, such as galls or leaf curling. Some diseases cause an underdevelopment of tissue that shows up as stunting, mottling, or incomplete development of organs. A third response is death of plant tissue, represented by blights, leaf spots, wilts, or cankers.

The Culprits

Most diseases are caused by fungi. Fungi are small, and it usually takes a microscope to see that they are composed of a mass of filaments called hyphae. They reproduce by spores. Fungi can infect the plant by directly penetrating the plant tissue or by entering through natural openings or wounds. Free water on the plant is usually needed for the fungal spore to germinate and infect the plant. Spores can be spread by rain, wind, insects, animals, and man. Fungi cause many different types of disease, such as fruit soft rots, stunting of the plant, smuts, rusts, leaf spots, wilting, and leaf curl.

Bacteria also cause plant disease. Bacteria are microscopic, one-celled organisms that mainly reproduce by cell division. They can build up in number very quickly in warm, wet weather. Most bacterial infections occur through wounds or natural openings such as stomates, hydathodes, or lenticels. The main diseases caused by bacteria are leaf spots or rots and wilts. The spread of bacteria from infected plants to healthy plants is by splashing water, insects, contaminated equipment, and in soil or plant debris.

Viruses, viroids, and phytoplasmas cannot even be seen with a regular microscope because they are so small. They are identified by the effect they have on the plant. A wound is needed for them to enter the plant. Viruses are made up of nucleic acid and a protein coat. There is no cellular structure, and it must reproduce inside another living organism. Many insects spread viruses; mainly aphids, whiteflies, and leafhoppers. Some viruses can be spread by nematodes or in infected plant pollen and by mechanical transmission such as grafting with infected plant material or using clippers that are contaminated. The plant responds to virus infection by exhibiting stunted growth, blotchy or mottled leaf color, or abnormal growth of plant parts.

Viroids are even simpler than viruses, consisting only of nucleic acid, and usually spread through infected plant stock. A few viroids have been found to be transmitted in pollen or in seeds. Phytoplasmas only live in the phloem, the nutrient transportation system of the plant, and cannot live outside of the plant or the insect host vector. Phloem-feeding insects such as leafhoppers are the vector for phytoplasmas.

The Controls

The goal of plant disease control is to prevent disease. There are many methods that can be used: resistant hosts, cultural control, mechanical control, sanitation, and chemical control. Using a resistant host breaks the disease cycle by removing a susceptible host for the pathogen to infect. Cultural controls alter the environmental conditions that are necessary for infection. Examples are crop rotation, use of metallized mulches, or controlling humidity levels in a greenhouse. An example of mechanical control is hot-water treatment of seeds and bulbs to control certain pathogens. Sanitation is a very important control that any type of operation can use, but is a very basic practice for organic operations. Examples are removing diseased plants to limit the spread, managing crop residue to prevent infection of the next crop, and disinfecting tools.

The primary disease control method that is used is the application of chemicals. While chemicals can be used for fungal and bacterial disease, there are no chemicals that control viruses, viroids, or phytoplasmas; however, chemicals can be used to control the insect vectors that spread them. For successful chemical control, the first objective is to correctly identify the pest. Then choose the correct chemical. Be sure to read and follow the label. Proper timing of the application and good spray coverage are very important. Also be sure to observe all reentry and preharvest intervals on the label.