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PLANT PATHOLOGY |
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KEY CONCEPTS
I. Definition of Plant Pathology
B. Disease cycle
1. Inoculation
III. Diagnosis of Plant Diseases
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DISEASE CYCLE
Often gardeners believe that their plants have become diseased overnight. This may be true in the case of damping-off. More often, however, much has occurred before symptoms are seen. There are five stages in disease development: inoculation, incubation, penetration, infection and symptoms.
The pathogen must be introduced (inoculated) to the host plant. Most pathogens cannot move on their own, but must be carried to the host plant. This is done by rain, wind, insects, birds and people.
Splashing rain carries spores of apple scab fungus from infected apple leaves to uninfected leaves. Wind blows fungal spores from plant to plant. The spotted cucumber beetle transmits bacterial wilt of cucumbers when feeding.
Working in the garden when plants are wet is a common way to spread disease. Disinfesting tools requires a 9-to-1 solution of water and bleach and takes a minimum of ten minutes. Smokers can transmit tobacco mosaic virus from a cigarette to tomato plants.
Seeds or cuttings from infected plants will also transmit disease. Certified seed guarantees that at the time of sale the seeds are free of all diseases. Seeds are often coated with a fungicide to prevent the transmission of surface fungal diseases.
Disease-free stock guarantees that the plant is not infected with disease. This is particularly important with perennial plants, such as roses, raspberries and other small fruits.
The second stage of disease development is incubation. The pathogen changes or grows into a form that can enter the new host plant. In many fungal diseases, the pathogen arrives on the plant as a spore which must germinate before it can grow into the plant.
The third stage is penetration or the point at which the pathogen actually enters the host plant. Once the fungal spore germinates, it sends out thread-like tubes call hyphae. These penetrate the plant through wounds or natural pores. Wounding roots of bedding plants during transplanting provides entry for root-rotting fungi. The mouthparts of an insect also result in openings for penetration.
The fourth stage is infection. The pathogen grows within the plant and begins damaging the plant tissue.
As the pathogen consumes nutrients, the plant reacts by showing symptoms. Symptoms are evidence of the pathogens causing damage to the plant. Symptoms include mottling, dwarfing, distortion, discoloration, wilting, and shriveling of any plant part.
Many pathogens can survive without a susceptible host even under the most unfavorable conditions. Many plant diseases survive from one growing season to the next on plant debris, seeds, alternate hosts or in soil.
Because of pathogen survival, it is important to remove and properly dispose of any infected plant materials. It is also important for the gardener to know about the diseases that affect each plant throughout the home landscape, as well as the conditions needed for proper culture.
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