(4) Examples of plant diseases and how they are spread

There several ways in which plant diseases (pathogens) spread.

Plant pathologists analyse the distribution of diseased plants because it helps identify the type of pathogen involved e.g.

isolated patches of disused plants suggest the disease is spread through the soil and entering the plant through the roots,

but, a random distribution of diseased plants suggests an airborne pathogen - a pathogen moved around at random by the movement of air and e.g. just settling on plant leaves.

Examples of the ways pathogens can spread

Some pathogens are carried in the air (airborne)

Erysiphe graminis is a fungus that causes barley mildew which produces white fluffy patches to appear on the leaves of barley plants. The powdery coating of the mildew reduces photosynthesis by reducing the light intensity and leads to a decrease in the crop yield. The fungus is spread by spores blown around from plant to plant by the wind.

Chalara ash dieback disease of ash trees is caused by the fungus hymenoscyphus fraxineus.

The fungus infects ash trees - the symptoms include black blotches on leaves and leaf loss (both reducing photosynthesis) and bark lesions leading to fluid bleeding.

In the majority of cases the tree dies, either directly or weakening so much it that the tree cannot defend itself against another pathogen.

The spores are transferred at random from infected trees to healthy trees by the movement of air - the wind.

It can also be spread when diseased ash trees are moved from one area to another - not a good move!

Direct contact between plant and pathogen

A plant can become diseased if it comes into contact with a surface contaminated with a pathogen.

The tobacco mosaic virus (TMV) attacks many species of plants including the tobacco, tomato, cucumber, pepper and some ornamental flower plant.

The tobacco mosaic virus gets into the plant through a lesion in the skin or epidermis of the plant.

The disease is caused by the tobacco mosaic virus which causes the infected leaves to become discoloured and mottled (hence described as a mosaic pattern).

The discolouration causes a decrease in photosynthesis, this inhibits growth and reduces the crop yield or quality of flower.

Other effects include curled leaves, stunted growth and yellow streaks or spots on leaves.

The tobacco mosaic virus is spread by infected leaves brushing against healthy leaves.

To control TMV you need to remove infected plants, sterilising tools at high temperatures, washing hands after handling infected material, possibly use pest control too.

Pathogens in the soil

Certain pathogens can live and thrive in soil and therefore plants can easily infected from the contaminated soil.

The bacteria Agrobacterium tumefaciens causes crown gall disease and spreads freely in soil and can actually grow on the roots of plants. The crown gall pathogen enters the plant through wounds in roots or stems and stimulates the plant tissues to grow in a disorganised way, producing swollen galls (tumor growths). The galls damage the plant tissue, restricting the flow of water in that part of the plant, weakening the plant as a whole and can cause it to die.

Rose black spot fungus

Rose black spot is a serious parasitic fungal disease that causes purple or black spots to develop on the stem and upper surface of leaves of rose and other plants and eventually the leaves turn yellow and drop off.

Rose black spot likes warm wet conditions to grow.

With less green upper surface leaf area, photosynthesis is reduced and thus reducing the growth of the plant.

The black spot fungus grows in the mesophyll and penetrates into the intercellular space and the plant cells themselves - damages cell membranes, causing nutrients to leak out into the intercellular spaces - this prevents photosynthesis, interrupting the food supply the plant needs.

Rose black spot fungus produces spores (act like seeds) and can spread through the environment by the movement of wind or water.

Gardeners treat this kind of disease in various ways:

(i) using fungicides  to kill fungus cells

(ii) stripping the affected leaves from the rose plant - the stripped leaves should be then destroyed to reduce the spread of the rose black spot fungus,

(iii) pruning shoots in the spring and burning all cut stems,

(iv) don't use infected leaves and stems in compost heaps,

(v) placing manure or mulch around the plants in spring to stop fungal spores getting to the stems.

Potato blight

Potato blight is another viral pathogen that attacks potato and tomato crops.

The potato blight virus causes the potato plants to rot and turn brown, the leaves shrivel, and the tubers decay.

Any infected potato plant material should be deeply buried, well below the depth of cultivation, removed to the local council green waste collection or burned, so best not to compost it.

In cultivation, earthing up potatoes well provides some protection to the tubers from blight spores washed down into the soil from lesions on the leaves or stems.

No fungicide is available at the moment to try and control potato blight.

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Keywords, phrases and learning objectives for this part on plant diseases and nutritional deficiency

Be able to describe and understand examples of plant diseases and how they are spread including fungus that causes barley mildew, chalara ash dieback disease, tobacco mosaic virus, other pathogens of bacterial infections in soil, rose black spot fungus and potato blight, a viral disease.

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