Controlling plant diseases

A plant's main enemies are pests and diseases. Most can be controlled using ecologically friendly methods. Here are the three types of diseases that could attack your plants.

Fungi are important agents in plant decay, and most work in the gardener's favour.

They help break down compost and are responsible for rotting wood in forests and returning the nutrients to the soil. Without them, the woods would be choked with dead trees.

Many fungal diseases are spread by very small spores that float on the breeze and land on a plant leaf. They "germinate" and insert a small thread (called a hyphae) into the plant tissue. This feeds on the plant and grows, forming a network of hyphae between the cells inside the leaf.

It is not until this point that the plant begins to show signs of stress — different-coloured spots, wilting, or fungal tissue visible (as in mildew). Such leaves should be picked off immediately and put in the garbage — not in the compost. This may be enough to stop the further spread of the disease.

Many plants, providing they are not under stress from poor growing conditions, can emit defensive secretions that can either kill fungal spores or limit the damage they can cause.

It therefore makes sense to give plants the soil conditions, acidity or alkalinity, and moisture in which they grow best.

Also, allow good air circulation through plants, especially those, like phlox and bergamot, that are prone to mildew. Stagnant air in the middle of a large clump of stems is a mildew heaven.

Other fungi are great opportunists and can gain entry through stem and leaf wounds caused by things like careless hoeing or mower damage.  Many cankers fall into this category.

Fungicides should be applied before the disease strikes to form a protective layer on the foliage that kills the spores on contact.

Bacteria are minute organisms that can be rod-shaped, spherical, or spiral. There are several million in five millilitres (a typical teaspoon) of soil. They are important in breaking down dead plant material, but a few, such as iris soft rot, attack living plants, generally causing plant tissue to disintegrate. They are difficult to control and long-lived in the soil.

Avoid replanting the same species in soil where a bacterial disease has been diagnosed. Some bacteria attack certain insects and are used as insecticides.

Viruses are submicroscopic primitive life forms that live inside cells of plants and animals. They tend to be very specific, limiting their attack to a single genus or plant family.

Some virus are used as insecticides but others attack plants. There is no cure and infected plants should be dug up and disposed of in the garbage. Infected plants usually have foliage with strange mottling or streaks, and are often puckering.

Plants' infections can be spread by hand, shears, and other gardening tools. Newly infected plants may take several years to show symptoms, during which time the virus can be spread to other plants.

Viruses are also spread by sap-sucking insects, such as leafhoppers, so controlling these insects is very important.

Many modern varieties of vegetables, especially tomatoes, have built-in resistance to some of the viruses and other diseases that attack them. This information is usually indicated in seed catalogues.

These guidelines will help you better understand the diseases that affect plants and how they can be treated.