PLANT PROTECTION 4

PLANT PROTECTION 4 – How to Diagnose Plant Problems
WHAT ARE THE CAUSES OF PLANT PROBLEMS?
Diagnosticians must have a good understanding of the causes of plant problems
pests, diseases and weeds
MAIN CAUSES
Insects are the
most numerous
and diverse type
of plant pest
Cabbage white butterfly
(Pieris rapae) – caterpillars
feed on cabbages, stock
Crown gall (Agrobacterium sp.)
– galls 20-300 mm across at
the crown of Rosaceous plants
PESTS AND DISEASES.
Because symptoms caused by insects and mites can be confused with those caused by plant
diseases, it is sometimes difficult to know whether one is dealing with a pest or disease (see
Fig. 1 below). Definitions of these terms are often inconsistent. In this book pests and
diseases have been grouped together and re-divided into 2 groups:
Parasitic pests and diseases caused by living agents (plants and animals), which
damage plants by obtaining food from them. Many can spread from sick to healthy
plants by a variety of means. Examples of parasitic pests and diseases include:
– Insects, eg cabbage white butterfly.
– Snails & slugs, eg common garden snail.
– Vertebrate pests, eg fruit bats.
– Nematode diseases, eg root knot.
– Virus & virus-like diseases, eg rose mosaic.
– Bacterial diseases, eg crown gall.
– Fungal diseases, eg black spot of rose.
– Parasitic flowering plants, eg mistletoe.
Non-parasitic pests and diseases include:
– Living agents (plants and animals) that damage plants mechanically, or in some
way other than by obtaining food from them, eg leafcutting bees, dogs, cats,
children, fairy rings, lichens, slime moulds. Arguably not a large group.
– Non-living agents that are not infectious and do not spread from affected to
healthy plants. They are almost infinite in number and include:
Environment, eg heat, moisture, light, transplant shock.
Nutritional deficiencies and excesses, eg iron deficiency.
Chemical injury, eg fertilizer burn, spray drift.
Mechanical injury, eg mower damage to trees and turf.
Genetic abnormalities, eg seed variation.
WEEDS.
Weeds, ie unwanted plants, are probably one of horticulture’s greatest problems.
Snails and slugs
damage a wide
range of plants
Black spot of rose
(Marsonnina rosae)
Fruit bats (flying foxes)
– can be pests of fruit
Mistletoe on a tree
Root knot nematode
(Meloidogyne spp.) –
galls up to 20 mm across
on roots of many plants
Mushrooms
growing on
organic matter
in turf
4 Background briefing – Causes of plant problems
Etiolated
seedling on
right – too little
light
Fig. 1. One example of each of the causes of plant problems.
Rose mosaic (a complex
of viruses) on roses
Winter grass, an
annual weed