DIS-EASE: Understanding the Signs & Symptoms of Stressed Plants by Jenny Slabber

DIS-EASE: Understanding the Signs & Symptoms of Stressed Plants by Jenny Slabber

A crying new born baby is bewildering to inexperienced parents, only until we learn to look to signs and symptoms to determine the cause of the discomfort and respond in the right way – if hungry feed, if nappy is dirty change it, if sleepy allow it to sleep. So if baby is hungry, putting it to sleep will not get to the root cause, nor will it resolve the crying! 

So too as new gardeners, we have to learn to look to the cause of pest and disease attack on our plants to solve problems causing DIS-EASE or stress conditions rather than to try to spray poisons year after year to deal with symptoms, without ever resolving the cause.

The most likely cause of pest and disease on plants are be the following:

  1. Incorrect watering – too much or too little, not adjusting for season and climate.
  2. Poor feeding practise – applying excess, or too little or not using the correct fertilizer balance for the plants requirements
  3. Soil structure and Aeration – sandy soil leeches water and fertilizer too quickly and heavy clay soils cause poor drainage and waterlogging.
  4. pH of soil – too low causes acidic soils and too high causes alkaline conditions, both extremes can inhibit uptake of nutrients.
  5. Plant positioning and grouping – sun lovers planted in shady spots, shade lovers planted in direct sun, high water requirement and low water plants grouped together.

Incorrect Watering

  • Too little water, hot & dry conditions

SYMPTOMS:  Powdery mildew, insect attack like mealy bug, Australian bug, scale and red spider mite. Wilting leaves and stems, eventual death.

Plants are made mostly of water, and like all other living creatures 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, transpiration which regulates plant 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 soil surface.

  • Too much water, hot & wet or cool & wet conditions

SYMPTOMS: Downy mildew, fungal disease like botrytis, black spot, rust, root rots, damping off disease and pests like aphids and white fly. Dead or brown leaf spots and blotches, yellowing of leaves, leaf drop, collapse of 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.

Powdery Mildew

Poor Fertilizer Practice

Plants, like people, need a healthy balanced diet and an adequate supply of all essential major, minor and micro-nutrients to optimise performance. Many pests and disease conditions are caused by poor quality fertilizers 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 re-applications of fertilizers 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, show mottled spots, interveinal patterns or scorched tips with reduced or no blooms and fruit.

SYMPTOMS: Excess or imbalanced fertilizers with high Nitrogen levels, result in fast and soft growth of leaves and stems that attract sucking pests such as Aphids and White fly.  Excess phosphate can lock-up and block uptake of micro-nutrients essential for plant functions by disturbing the soil chemistry resulting in slow growth.

REMEDY:  Select quality organic fertilizers which contain the full spectrum of plant nutrients best suited for 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 are seldom nutritious enough to ensure healthy and productive plants in our low nutrient garden soils. 

Soil Structure & Aeration

Soil structure has a major effect on a plants resistance to stress and disease and ability to grow with vigor.

Sandy soil structure: Water and nutrient retention is poor leading to plant stress. 

SYMPTOMS: Plants droop and wilt easily, colour is sickly light green or yellow, deficiency symptoms on leaves, interveinal patterns and spots. Powdery mildew and wilting disease. Pests include red spider mites, aphids, scales, mealy and Australian bugs, 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 which 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 pH & soil Chemistry

Nothing grows well in soils that are excessively acid or alkaline, even plants that have a natural preference for either, like fynbos, Protea 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 result in deficiency diseases. When 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 acid soils regular addition of Dolomitic lime will gradually change 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 & Posistioning in the Landscape

Plants that are grouped together inappropriately will seldom be healthy and therefore prone to stress which 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.

Deficiency chart of Micronutrients

Talborne Organic Products available in our store at Garden World!