Understanding the Signs & Symptoms of Stressed Plants
by Jenny Slabber
A crying new-born baby is bewildering to inexperienced parents, only till we learn to look to signs and symptoms to determine the cause of the discomfort and respond in the right way – if the baby is hungry feed it, if the nappy is dirty change it, if the baby is sleepy allow it to sleep. So if your baby is hungry, putting it to sleep will not get to the root cause, nor will it resolve the crying!
So too do new gardeners have to learn to look to the cause of pest and disease attack on their plants to solve problems causing disease or stress conditions, rather than trying to spray poisons year after year to deal with symptoms, without ever resolving the cause.
The most likely causes of pest and disease on plants are the following:
- Incorrect watering – Too much or too little, not adjusting for season and climate.
- Poor feeding practise – Applying too much or too little fertiliser, or not using the correct balance for the plant’s requirements
- Soil structure and aeration – Sandy soil leeches water and fertiliser too quickly and heavy clay soils cause poor drainage and waterlogging.
- The pH of the soil – Extremes in the pH levels, such as low pH (acidic soils) and high pH (alkaline soils), inhibit the uptake of nutrients.
- Plant positioning and grouping – Examples are sun lovers planted in shady spots or shade lovers planted in direct sunlight, or high-water-requirement and low-water-requirement plants grouped together.
- Too little water, or hot and dry conditions
SYMPTOMS: Powdery mildew, insect attacks from mealybug, Australian bug, scale and red spider mite. Wilting leaves and stems, eventual death.
Plants are made mostly of water, and like all other living organisms they are absolutely dependant on a regular and adequate supply of water, which is essential for processes like hydration, carrying nutrients to all parts of plant tissue and transpiration, which regulates a plant’s temperature.
REMEDY: Water more often and for longer periods to ensure a reserve of water and prevent plant stress. Add water-retention products and compost. Mulch the soil surface.
- Too much water, hot and wet or cool and wet conditions
SYMPTOMS: Downy mildew and fungal disease like botrytis, black spot, rust, root rot, and damping off disease, and pests like aphids and white fly. Dead or brown leaf spots and blotches, yellowing of leaves, leaf drop, collapse of the plant and roots blacken and die off.
Too much moisture leads to rotting and fungal disease.
REMEDY: Reduce watering in rainy periods or in cooler months when less water is lost through transpiration (like people sweat to regulate body temperature) in leaves. Ensure adequate drainage in beds and pots. Add compost and water-holding products to absorb and hold excess water.
Poor fertiliser practise
- Plants, like people, need a healthy balanced diet and an adequate supply of all essential major, minor and micronutrients to optimise performance. Many pests and disease conditions are caused by poor quality fertilisers that might be cheap but do not satisfy the full requirements of the plant, or leach out very easily with watering or rain, and the nutrients are only available for plant uptake while they pass the root zone. Therefore, very frequent applications of fertilisers are necessary to adequately feed the plant, or nutritional stress develops. Low nutrient levels or too rich and fast a supply can also bait in pests and cause disease.
SYMPTOMS: Plants deficient in nutrients have soft growth, like drooping leaves and stems. Leaves that yellow or that show mottled spots, interveinal patterns or scorched tips with reduced or no blooms and fruit.
SYMPTOMS: Excess or imbalanced fertilisers with high nitrogen levels result in fast and soft growth of leaves and stems that attract sucking pests such as aphids and whitefly. Excess phosphate can lock-up and block uptake of micronutrients essential for plant functions by disturbing the soil chemistry resulting in slow growth.
REMEDY: Select quality organic fertilisers that contain the full spectrum of plant nutrients best suited for the growth phase, like planting for healthy root and plant structural development, or green leafy growth or to encourage flowering and fruiting. Compost or manure on their own is seldom nutritious enough to ensure healthy and productive plants in our low-nutrient garden soils.
Soil structure and aeration
- Soil structure has a major effect on a plants resistance to stress and disease, and its ability to grow with vigour.
Sandy soil structure: Water and nutrient retention is poor leading to plant stress.
SYMPTOMS: Plants droop and wilt easily, their colour is sickly light green or yellow, their deficiency symptoms on leaves, interveinal patterns and spots. Powdery mildew and wilting disease. Pests include red spider mites, aphids, scales, mealybugs, Australian bugs, and ant activity.
Tight clay structure: Poor root development and penetration into soil.
SYMPTOMS: Root rots cause collapse and death in plants. Crickets thrive in clay soils and release methane gas as they eat rotting roots and plant material. Oedema (lesions and cracked blisters on leaves often seen on brassica family) and cracked fruits such as tomatoes with waterlogging in soils where drainage is inadequate.
REMEDY: Prepare soils well before planting by adding lots of compost, earthworm castings and water-retention soil conditioners to improve soil structure, friability, water-holding capacity and aeration to prevent disease and pest attack.
Adjust the pH and soil chemistry
Nothing grows well in soils that are excessively acidic or alkaline, even plants that have a natural preference for either, like fynbos, proteas and blueberries that prefer slightly acidic soils and Mediterranean crops like olives, grapes and lavender that like limey or alkaline soil conditions. Any extreme tends to lock-up nutrients, which results in deficiency diseases. When the pH is corrected the chemistry and nutrient uptake resumes, and the soil ecosystem is healthy and active.
SYMPTOMS: Unproductive plants vulnerable to pest and disease. Excess weed pressure.
REMEDY: Condition soils with good quality compost. To remediate acidic soils the regular addition of dolomitic lime will gradually change the pH. Adjust Alkaline soils by working in acidic compost or application of small quantities of elemental sulphur (also known as flowers of sulphur) at a time can be applied and leached into soil by watering.
Incorrect plant groupings and positioning in the landscape
Plants that are grouped together inappropriately will seldom be healthy and are therefore prone to stress, and so will be constantly threatened by pest and disease.
REMEDY: Plan and research the ideal conditions of plants used in the landscape according to their light and water requirements.