Ascochyta sorghi causing leaf spot on sorghum. Source: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org, image 1436039
plant disease stages
By the time you notice disease symptoms on your plants, the pathogen (disease organism) has already gone through four stages unnoticed. The development of symptoms is the last of five stages in disease development: inoculation, incubation, penetration, infection and symptoms.
Understanding these stages and allowing for them in your management plans can reduce the incidence of disease in your crop or turf.
Inoculation
Before a pathogen can infect a plant, it must be introduced to the plant. This process is called inoculation. Most pathogens rely on rain, wind, insects or people to carry them to their host (although some, such as Phytophthora, can lurk in the soil). For example, splashing by rain carries spores of the apple scab fungus from infected apple leaves to uninfected leaves. Wind blows fungal and bacterial spores from plant to plant. Aphids transmit viruses when feeding. People frequently spread diseases on their clothing, footwear or tools. In particular, smokers can transmit tobacco mosaic virus to many different plants, as tobacco is commonly infected.
Several techniques can reduce the chances of inoculation:
- Work in plants when they are dry, as water favours spread.
- Sterilise your tools in bleach diluted 1:9 or in alcohol diluted 1:1.
- Buy certified disease-free seed or plants (critical for potatoes and strawberries).
- Sow fungicide-treated seed.
- Grow disease-resistant cultivars.
- Wash your clothes between crops or fields.
- Sterilise your footwear in formalin solution.
- Control insects that might transmit disease. Integrated pest and disease management (“IPDM”) can achieve this at less cost than regular spraying.
- Rotate crops to allow enough time for the disease to die in the absence of its host.
Incubation
Before a pathogen can enter a plant, it must incubate first. This stage applies to fungi and bacteria, although not to viruses. In many fungal diseases, the pathogen arrives on the plant as a spore, which must germinate before it can enter the plant. The incubation period depends on temperature (shorter in warm weather), moisture (moisture encourages it) and perhaps light (as UV in sunlight can destroy some pathogens).
Penetration
Penetration is the stage at which the pathogen actually enters the host plant. In the cases of insect-borne viruses, the insect’s piercing mouth parts pierce the plant tissue, and the virus is injected (unintentionally, of course). In the case of fungi, once the spore germinates, it sends out thread-like tubes that penetrate the plant through wounds or breathing pores (the stomata), or in some cases by dissolving a hole. Any activity that could wound the leaves, stems or roots could provide entry for fungi and bacteria.
Infection
Finally the plant is infected. The pathogen grows within the plant and begins damaging the tissue. In some cases the damage is due to active decomposition of the plant tissue. In other cases the physical presence of the pathogen within the plant can block the water-conductive tissues. And in some cases the pathogen robs the plant of its nutrients or sugars. In the case of viruses, the virus hijacks the plant cells to produce more viruses instead of more plant.
Symptoms
Finally, possibly days after inoculation, the plant shows symptoms, which can include mottling, stunting, distortion, discoloration, wilting and shrivelling. Later symptoms can include reduced yield, failure to bear and even death.
Disease identification and control
By the time symptoms appear, control is urgent. Identifying the disease can be critical to successful control. Many guides are available from state and territory departments of agriculture or primary industries to help you identify the organisms responsible and the recommended control measures. Alternatively, if you are not confident of an accurate identification or the problem is urgent, you can send us some samples for our 24-hour-turnaround disease identification service.
Further information
NSW I&I (Primary Industries): Pests, diseases and disorders in horticultural crops.
NSW I&I (Primary Industries): Pests, diseases and disorders in field crops and pastures.