Gardening: Stress can kill, infections and disease

Almost every time I explain to someone that they should avoid causing a plant stress, I invariably get a certain look. The one where you just know that the person you are talking to thinks you have been spending far too long conversing with vegetation. As a keen horticulturist I remain undaunted, as this is one area where our job is similar to that of the medical profession: we have a duty to educate as many people as possible so that a plant here or there might live a better life.

Now I will accept that in some cases trees and shrubs die without obvious cause, and while one can try to determine the cause of death posthumously, this is not at all easy. Indeed, I am sometimes thrust a corpse and asked expectantly for the cause of death, but it's the symptoms of a plants suffering that tell us the story, not the end result. I can mimic a doctor - but not a psychic.

One major area that can cause confusion is secondary infection which, in another close parallel to human health, can lead to misdiagnosis. Where trees and shrubs are under stress or have been damaged, often one will see other insects or diseases install themselves. These can at times be more outwardly obvious than the initial problem, making replanting risky as the original problem may still be present. Stresses may be natural or man-made but once again, as in humans, they have similar descriptions, such as chronic (long-lasting, repeated) or acute (immediately serious). Waterlogging by over-watering is an example of a chronic problem while frost can cause acute issues. It is so important to look carefully at why a particular plant is looking sad and try to understand the reasons for this.

All woody plants have certain reserves and in most cases will not immediately show any obvious symptoms. However, little by little the reserves are depleted and signs will inevitably become evident. Physical damage is often the easiest to spot, with damage to trunks or stems hindering the passage of nutrients and resulting in die-back of growing points (shoots or roots). This is one of the best places to look for signs of problems, especially in large trees which have substantial reserves: a tree that appears otherwise healthy will often show a problem first at the centre and very top of the crown.

Other less obvious but very common causes of stress are numerous, some of which I have covered in previous articles: over-watering, under-watering, erratic watering, poor drainage, pest infestation, disease infection, soil compaction, poor nutrition and mineral depletion are some of the usual suspects. As these stresses are applied, plants often fall victim to a downward spiral as they show symptoms: with time, fewer and fewer leaves are produced, leaving the plant unable to produce enough energy to grow and renew its roots, while the lack of leaves can leave the stems exposed to sun damage and overheating. With a decaying root system, the symptoms will accentuate, with one effect feeding the other. Leaves will be fewer and smaller in size and will often be discoloured. Some plants will, in a last ditch effort to perpetuate their genetic line, produce prolific fruit at the expense of their last remaining reserves. Lemon trees are often seen to do this, during which they will produce little or no leaves and recovery after this is unlikely. Lastly, if these cycles do not directly kill the plant then it will certainly be weakened, allowing secondary infection by what are known as opportunistic diseases and pests. The combination will usually result in the prompt demise of an already weakened plant.

It is not all bad news though, as being aware of these issues allows you to address problems, work to prevent stress and provide optimal growing conditions for your plants. This can sometimes start even before planting, by making judicious choices for plants for a particular location based on the conditions there and what a particular plant is adapted to. This can at least avoid an uphill struggle. (Again, citrus trees planted in the centre of an irrigated lawn jump to mind as an example this.)  

Some do's and don'ts that fall outside the obvious need to feed and water correctly: Try to avoid damaging tree roots by compaction if you are doing any construction work, especially with heavy machinery. Create a "no-go" zone just inside the canopy line to be sure of this if the tree is valuable. On an ongoing basis, avoid any kind of physical damage to the base of the trunk or stem of the tree as this is irreparable and can be fatal. I often see lawnmower or strimmer damage to the bases of trees or shrubs planted in lawns. If the stem becomes "ringed" - where the bark is stripped off all the way around - it's always fatal. Avoid over-fertilising with excessive feed in the spring at this can lead to decline in the summer as the plant is unable to supply water and nutrients fast enough to the lush stem and leaf growth it has produced. Equally, be wary of plants that have been grown in this manner and look "forced" - they grow fast and are therefore lower cost, but as with most things, not always the good deal that they first appear.

Now, if I can just remember where I put my prescription pad... did you remember to bring your carte vitale?  
CLIPPINGS: Avoid unnecessarily depleting the natural reserves of a plant by harshly pruning after the main push of growth in the spring. This will leave a plant with less built-in ability to resist for any duration should a problem arise during the season, and reduce the time you have to correct the problem.  J.H.

From Riviera Reporter 124, Dec 2007/Jan 2008