More than one method to control plant disease

Happy Gardening
by William H. Bohl

You can’t operate a vehicle without gasoline, which is just one of many essential elements needed to make a vehicle run. It also needs oil, spark plugs, etc. Leave out any one of the essential elements and the vehicle fails to operate. Occurrence of a plant disease can be similar to keeping a vehicle working.

When plants don’t grow normally, they can be considered to be diseased. A very broad definition of plant disease includes anything that interferes with normal plant development. Using that definition, non-living environmental conditions, such as excessively high temperature, could be considered disease-causing factors. Most often, however, disruption of plant growth is caused by a living organism, which includes fungi, bacteria and viruses. Technically viruses aren’t living but need a living host to survive. Collectively, these organisms are called pathogens.

Plants cannot become diseased unless three factors come together. There must be a susceptible host plant, a pathogen capable of causing a disease, and a favorable environment. Like the vehicle example above, eliminate one of these factors and a disease will not develop. Pathologists commonly relate the relationship of these three factors to the sides of a triangle.

One side of the disease triangle is a host plant. Only those plants susceptible to a particular pathogen can be infected. For example, fireblight can only infect plants in the rose family such as apples and pears.

Another side of the triangle is a virulent pathogen. A pathogen must be in a stage of development capable of infecting and in sufficient quantity to cause disease. For example, the white mold organism that infects many plants goes through a resting stage in the soil. During this resting stage, the organism cannot transmit the disease to a host plant.

Most diseases have fairly specific environmental conditions that must be met for the pathogen to infect a plant, the third side of the disease triangle. Environmental conditions include such factors as appropriate temperature and moisture. For example, a common fungal disease of tomato and potato is early blight. Conditions favoring disease development are alternating wet and dry conditions and a temperature of 68 degrees.

Oftentimes gardeners think a chemical is needed to control a plant disease but there are other methods as well. Think about altering one side of the disease triangle. For example, many leaf diseases require moisture to develop, so keeping the foliage dry dramatically minimizes disease development. Also, consider planting resistant varieties for controlling diseases that are a perennial problem such as Verticillim wilt of tomato. Using an integrated disease management program considering all sides of the disease triangle will help reduce losses from diseases.

Happy Gardening!

Bohl is Extension Educator with University of Idaho located in Blackfoot. He may be contacted at 785-8060 or


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