Common Pests and Diseases of Plants

Pests

• Aphid

• Caterpillars

• Scale

• Thrips

• Whitefly

• Red Spider Mite or Two-spotted Mite

• And many others

Aphids

• Aphids, semi-transparent insects 5mm long that cluster on new growth and flower buds.

• They suck the juice from the plant and secrete the sticky substance

• Aphids can carry and spread diseases.

• Aphids may be knocked off plants with a stream of water

• In severe infestations, spray with an insecticide or an insecticidal soap

Caterpillars

• Pick off and squash.

• Use Dipel which is a biological agent that affects caterpillars only.

• Pyrethrum may also be effective.

Scale

• Looks like small limpets stuck on to leaves, often on the midrib

• White Wax Scale looks like blobs of white wax

• May be washed off with soapy water

• Spray with white oil

Thrips

• Thrips are very small and hard to see

• They are light brown, slender insects, who while in the adult stage will fly to other plants when disturbed

• They 'rasp' into the leaves and flowers to obtain the plants juices, leaving the leaf distorted, with noticeable scars

• Spraying with insecticidal soap will usually solve the problem.

Whiteflies

• Whiteflies are tiny, snow-white insect pests that (when viewed under a magnifying glass) resemble moths.

• When viewed without magnification, these insects look more like flying dandruff!

• They attack a wide range of plants including bedding plants, cotton, strawberries, vegetables, and poinsettias

White wax scale

• So called because of the white waxy covering which is so evident in the background image

• Effects – disfigure plant, slowing up the growth of the plant

• Pruning is one solution or brush off with broom

• Otherwise spray with white oil

• Another option is washing soda.

Red Spider Mite or Two-spotted Mite • Tiny and only just visible to the naked eye

• They may be bright red but are often greenish or yellowish

• They are usually found on the undersides of leaves

• They like dry conditions so spraying up under the foliage & keeping the area well-watered and humid will discourage them.

Plant Diseases

• The term "plant disease" refers to anything - except insect damage - which may affect the growth of a plant, cause abnormal growth or death.

• Included, therefore, are parasitic organisms, unsuitable soil, incorrect temperature, injury from fumes and sprays or excessive liming.

Plant diseases may be divided into two main groups

• Disorders or non-parasitic diseases, which are not infectious

• and parasitic diseases, which are infectious

Parasitic diseases

• fungal and bacterial diseases

• Viral diseases.

Fungal Diseases

• Damping-off • Phytophthora root rot • Downy Mildew • Powdery Mildew • Black Spot • Rust • Peach Leaf Curl • Collar Rot • And many others

Fungal diseases

General rule, to help prevent fungal disease

• avoid over-head watering,

• give plants plenty of space,

• increase air circulation around plants

• ensure that the soil drains well

• remove and burn or dispose of affected leaves or plants to help to prevent the spores spreading

Powdery Mildew

• A variety of fungi producing white or light grey, powdery coating leaves, buds, flowers. A powdery mildew beginning with white spots

• Humid, overcast weather, especially when the days are warm and the nights are cool and moist are prime conditions for the fungal disease Powdery Mildew (Oidium sp.)

Powdery mildew

• Powdery mildew is one of the most common and easily recognized plant diseases

• Almost no type of plant is immune, however some are more susceptible than others.

• Lilacs, crab apples, phlox, roses, grapes, squash and cucumbers are all likely targets for powdery mildew.

Powdery Mildew on Rose. Note distortion of some leaflets.

Botrytis - Grey Mould

• Also occurs in cooler months and appears as a grey furry growth, leaving brown spots on the leaves

• This fungus is spread by spores held on decaying plant material

• Keeping your shadehouse tidy and well ventilated is the best preventative for this fungus.

• Sprays can also be used if needed.

Damping Off

• Most commonly occurs when germinating seeds in trays under heated or sheltered conditions.

• This fungus spreads through soil water and can devastate whole crops if not detected quickly.

• There are two main forms - 'pre-emergence' damping off when the seeds rot before sprouting, or the more common form of 'post emergence' where the seeds sprouts, the shoots emerge, then rot at soil level and fall over.

Black Spot of Rose

• The common name of this fungus disease says it all as far as what it looks like

• It spreads by releasing spores, which are carried on air currents

Rust

• produces pales greyish areas on the leaves and calyces

• These burst open to expose masses of reddish-brown spores which spread by wind.

Rose ( Rosa ) rust on the underside of a leaf.

Gerbera 'rust' on the underside of a leaf.

Snapdragon ( Antirrhinum ) rust on the lower leaf surface (magnified).

Bacterial diseases

• Crown Gall (especially stone fruits & roses)

• Bacterial Canker (of stone fruit, citrus, hibiscus, tomato & capsicum)

• Bacterial Soft Rot (of carrot, potato, onion, iris)

• though there are many others.

• A bacterial leaf disease on Asplenium (Bird's Nest Fern). Translucent lesions appear around the leaf margin.

• Older lesions have turned brown.

• Bacteria were isolated from the rectangular cut in the leaf margin.

Bacterial canker

• Bacterial canker is often observed on young trees starting to bear fruit.

• Note the extensive gumming along the trunk.

This bacterium can also cause spots on fruit and leaves

Bacterial Soft Rot

• This bacterium is commonly found in soil and plant debris, and it helps speed up plant decomposition.

Bacterial soft rot can be devastating for Iris

Prevent Bacterial diseases

• Conditions favouring fungal disease also favour bacterial infection & the same preventative measures apply .As well as:

• Use sharp pruning tools. • Don't prune in wet weather. • Don't damage tree bark with whipper-snippers or

mowers. • Avoid bringing in soil. • Buy plants and seeds from reputable suppliers.

Viral diseases

• Viral diseases are most often observed by unusual marking and colouration of leaves and sometimes petals - patterns, mottling, mosaics, streaking, ringspots - and malformation of leaves, flowers or fruit.

Viral diseases

• Some plant viruses don't cause much damage, but most stunt the plant, reduce its lifespan or kill it outright.

• There is no cure

Viral diseases

• Viruses are most commonly spread through the action of sap-sucking insects such as aphids and leafhoppers, so controlling these insects is important.

• Failure to clean pruning equipment and hands may also transfer viruses in plant sap

• Plants affected by serious viruses should be destroyed and removed.

References

• http://vegetablemdonline.ppath.cornell.edu/PhotoPages/CropHosts/Alfalfa.htm

• SINCLAIR STAMMERS/SCIENCE PHOTO LIBRARY