Common Plant Diseases - description and treatment

Down Garden Services

PLANT DISEASES

Use the links on the right to view details of some common plant diseases. The link at the top is for pictures of some of the diseases.

Plants can suffer from bacterial, viral and fungal attack just as we can ourselves. The organisms themselves (pathogens) are different, but at the microbial level the infection is much the same since one cell is as good a host as another. No matter which part of the plant is attacked the effect is usually to weaken or kill it. By infecting the leaves the plant's ability to produce its food is reduced. Some pathogens block the vessels in the stems which supply the leaves and by attacking the roots, the uptake of water and nutrients is reduced or stopped completely.
When a plant is attacked by one of these microorganisms the damage caused provides an opportunity for the others to get in and it is the combined onslaught which deals the final blow. Also if it is under stress, such as through drought or poor nutrition it is more susceptible.

Sometimes the 'infection' is symbiotic where both organisms derive a benifit. A good example of this is the nitrogen fixing bacteria ( Rhizobium ) which reside in nodules on the roots of leguminous (pea family) plants, the plant provides food and protection, the bacteria takes nitrogen from the air and converts it to a form usable by the host. Also the Mycorrhizae are a whole Order of fungi which have a symbiotic relationship with plant roots.

In other cases of interest to gardeners, the plant is not benefited but the changes caused produce more attractive features. This is what happens with Aucuba japonica where a viral infection produces the mottled leaves in the 'Variegata' variety. The Tulipomania craze in the 17th century was caused when 'Broken' varieties of tulips began to appear with streaked and mottled petals. This occurred at random and increased the desire and fascination for the bizarre effects. Prized examples were valued at more than "a mansion with servants", although it was not discovered until the early 20th century that viruses were to blame for the unusual colours and effects. In Holland, where the craze caused great hardship when it crashed in 1637, they now grow the pure forms with no streaks or frills.

Fungi are essential in breaking down dead organic matter to produce the humus which is needed for good soil structure - saprophytes. They do not have any chlorophyll so cannot use light to capture energy, instead they derive their energy by breaking down plant and animal material - alive or dead. They can also live in a symbiotic relationship, eg. the micorrhiza in the fine roots of conifers which cannot survive without them to take up vital nutrients. The widespread use of chemical control can damage the balance of these beneficial fungi and this forms part of the principals of Organic management. Lichens are an algae and a fungus growing together as a symbiotic conjunction, ie. the fungus provides physical support for the algae and the algae produces food.
There are some less welcome fungi which attack living plants and weaken or kill them - these are the ones which are mentioned in more detail here.

Viruses dwell inside the cells and cannot be treated with chemicals so affected plants must be destroyed (special microculture techniques may overcome the infection by taking cells from the growing tip, but this is restricted to the laboratory). There are no antibiotics for plants, so bacterial attacks, eg. fireblight, are untreatable as well. Fungi can be killed with chemicals without damaging the host because their growth habit is different, ie. they tend to grow on the plant and not in it, using root-like structures to extract nurishment.

Since killing the pathogens is difficult or impossible, "prevention is better than cure". By observing good hygiene when propogating and growing your plants, you can prevent a lot of diseases from taking hold.

Destroy diseased plants, clear up dead leaves and other debris.

Prune fruit trees and bushes regularly to keep an open structure allowing a good air flow and to remove damaged branches.

Disinfect secateurs, saws or knives used for cutting out diseased branches with methylated spirits or a flame (a cigarette lighter comes in handy for this). This also helps when taking cuttings.

Only use new or well-washed containers when growing cuttings and sowing seeds.

Crop rotation in the vegetable plot will prevent a build up of disease.

Space plants well apart especially crops where similar plants are growing together, to allow good air flow. Fungal diseases in particular, thrive in still, damp air and there is a greater chance for them to be transmitted to surrounding plants if they are in close proximity.

Catching disease early is important so keep an eye out for it at all times.

Plants are more susceptible to disease if they are not growing well. This can be due to poor soil, drought or both. So prepare the site well adding plenty of organic matter and give plants an occasional feed. As gardeners we are usually trying to grow plants which most likely are not native to the region or in a place they would not choose for themselves, so there is a greater probability that they could be under stress.

Of course not all plant diseases can be prevented by good hygiene as some are transmitted by insects and others are wind-borne. Aphids and other sap-sucking insects are the main vectors of viruses. Tobacco mosaic virus can be harboured in cigarettes, so smokers are banned from greenhouses where Surfinia Petunias (susceptible to the virus) are raised from cuttings - all stocks had to be destroyed in 1995 when they became infected. The spores of fungal diseases are carried in the air, and in rain drops and splashes.
Be careful when bringing plant material into the garden. Diseases can be in the soil as well as the plants themselves. Some gardeners will only raise their plants from seed, especially vegetables as some onion sets may have White Rot or brassicas may have Clubroot and this would infest the plot for years to come.