The A-Z Plant Health Handbook for New Zealand,
   by Jon Muller.

CAUSES OF PLANT PROBLEMS

The three main causes of plant problems are pests, diseases and disorders, which we will discuss below.

Pests

(Excerpts)

There are two main groups of pests, based on how they cause damage. They are chewing pests and sucking pests.

1. Chewing pests such as caterpillars, which are the larval stage of butterflies or moths, chew holes in the leaves. Grass grubs, which are the larval stage of beetles, eat the roots of the turf, causing die-back. Adult beetles are the other major group of chewing pests.
   Lemon tree borers or leaf miners chew their way through the inside of stems and leaves. This can cause die-back in the case of the lemon tree borer, which also affects other plants. On plants such as kaka beak, leaf miners cause unsightly tunnels in the leaves.

   Fig. 2 Grass grubs are the larval stage of a beetle.

2. Sucking and rasping pests include aphids, which suck the plant sap from inside the leaf, or thrips, which rasp and then suck the sap from the underside of the leaves. These pests can congregate on leaves, stems or even roots. They typically cause distortion, such as the puckering of by psyllids on karo leaves, or silvering of leaves as the sap from the leaf surface is removed by thrips.

Diseases

(Excerpts)

There are three main groups of pathogens or disease-causing organisms that cause a range of symptoms. They are fungi, bacteria and viruses. These are all microscopic organisms, but the effect they can have on plants is very large compared to their size. They can reproduce and spread very quickly.

1. Fungal diseases include cankers, damping off of seedlings, mildews, galls, moulds, leaf spots, rusts, rots and wilts. Once a plant is infected with a fungus, it is often difficult to eradicate it.
2. Bacterial diseases include blights; leaf, flower and fruit spot; canker; rot; and die-back.
3. Viruses are tiny, non-living particles that infect plant cells. They cannot be controlled directly by chemicals. They commonly cause stunting, mosaics, distortion, chlorosis or yellowing, poor flowering and fruiting, and streaking.
   You can buy 'High Health' plants that are free of all known viruses when sold, but they are not resistant to infection later. Roses and daphne are two examples of 'High Health' plants that can be purchased in garden centres.

Fig. 5 Black spot on a rose

Disorders

(Excerpts)

A disorder is a plant problem not caused by a pathogen or pest. Examples of plant disorders are nutrient deficiencies and environmental factors such as frost damage. We will look at some common disorders below.

Nutrients

Plants require 16 essential elements or nutrients for growth, such as nitrogen, phosphorus and potassium. Each of these elements is needed in the correct form and amount. If any one element is in short supply, it becomes the limiting factor in plant growth.

Environmental factors

Environmental factors such as light, temperature, moisture and wind also affect plant growth. As noted earlier, each plant has an ideal set of conditions. If one of these factors is not ideal it can result in poor growth. For example, planting a sun-loving plant in the shade can make it soft and drawn, while planting a shade-loving plant in the sun can cause scorching or yellowing. Strong, salt-laden wind and frost can cause similar physical damage to plants, even though the environmental conditions are quite different.
Do not be tempted to jump to conclusions. A checklist such as the one below can be useful in deciding the cause of the problem.

Checklist for plant problems

(Excerpts)

• What is the plant?
• Extent of the problem: is it all over the plant or just in places? Is it on new or old growth? If the new growth is healthy then the symptoms on old leaves may not be relevant.
• Timing of the problem: has it occurred recently, or been there a long time?
• Appearance of the problem-regular checks of plants can detect early symptoms.
  Check anything that looks different from healthy plants, such as wilting, yellowing, holes in the leaves or spots. You may note pests on close inspection, but be aware of anything that catches your eye. Check the roots as well as the parts above ground. To check the roots you will need to remove the soil carefully from around the roots. Look for any signs of rot, die-back or discoloration.
• What is the drainage like? Has there been flooding lately near plants that prefer good drainage?

Remember that you need to consider the ideal growing conditions for the plant initially when considering the above factors. Once you have established the likely cause of the plant problem, you can look at some solutions. The first solution should be to ensure the plant is growing in its ideal environment. Try to provide this, or even shift the plant if this is not possible. If the plant cannot be provided with these conditions, and control is difficult, then it may be better to remove it and plant something more suitable.

Excerpts

Looking after Roses & Camellias

List of plants included

Key to problems

Causes & Solutions

Guide to Chemicals
&
Alternatives to Chemicals

Introduction

Table of Contents

Links

Buy books

Acknowledgement