WHAT IS WRONG WITH MY ROSES?
Whenever one thinks that the roses in a garden are not growing well or do not look healthy or even abnormal it is best to correctly identify the problem and the cause before attempting to spray with a pesticide. Very often it is a growing problem that reduces the natural resistance to insects and diseases. The illustrations below should help in the identification.
Rose leaves inform the observant gardener of the plant’s wellbeing or of its problems. Large, deep green leaves indicate an active root system, well aerated, with enough moving water and adequate nutrients available for absorption. Stem length and flower size can be expected to be superlative. Smaller leaves can indicate heat stress and insufficient water, while discolouration of the leaves shows a deficiency of nutrients – see section on fertilising.
The most common, clearly noticeable deficiencies are of nitrogen and iron.
Nitrogen deficiency is indicated by pale green-yellow leaves and shoots. On an average rose bush, approximately 20 to 30 actively growing shoots extend, in total, by about 5cm per day. This growth requires a considerable amount of nitrogen. Being water soluble, nitrogen is leached out of the soil more quickly than any other nutrient, particularly in sandy soil. The only way to bring a plant out of such a deficiency is by adding a fertiliser high in nitrogen (N). Within 10 days of application and watering a distinct difference in the greening of the leaves and sprouting should be evident. Using manure takes twice as long to bring about a change and with well rotten compost even longer.
It is indicated where leaf veins remain green, but the areas in between gradually turn light green to yellow, and almost white in severe cases. An iron deficiency does not necessarily mean that there is no iron in the soil, but rather that it cannot be absorbed by the roots – usually the result of insufficient aeration due to soil compaction, over-watering or waterlogging. Without a good supply of oxygen in the root zone, micro-organisms are inactive and do not convert iron, manganese, boron, copper, sulphur and other essential trace elements into a form that can be absorbed by the roots.
Sudden loss of leaves (leaf drop caused by disease, removal of too many active leaves when cutting long-stemmed blooms, or hail) disturbs the balanced flow of sap, leaving the roots unable to absorb and transport trace elements. Signs of deficiency become apparent in the leaves within days.
When the soil contains relatively high percentages of lime or sodium (natrium combinations), causing high alkalinity, absorption of some micronutrients, including iron, is blocked. Acid organic material such as peat moss and milled pine bark helps regulate this problem, together with a handful of flower of sulphur sprinkled around each plant once or twice a year. (The condition will compound if caused by alkaline irrigation water.)
Various deficiencies occur due to a very high pH of 8.5. This high pH can be reduced by applying a good handful of flour of sulfur around each bush, and then watering it in.
A magnesium deficiency can be overcome by spreading Epsom salts around the bushes or by using Ludwig’s Vigorosa® fertiliser.
TO RECTIFY DEFICIENCIES
Ensure adequate aeration of the soil, and treat plants with trace elements in chelate form (for easy absorption by the leaves and roots), or add them to the spray cocktail. The most popular commercial product is Phostrogen, which contains all required nutrients. To correct deficiencies almost immediately, wet the leaves and drench the soil with 50g of Phostrogen in 10litres of water for about ten medium-sized bushes.
A common practice is the annual application of a tablespoon of Epsom salts (magnesium sulphate). Magnesium is an important trace element, responsible for the formation of chlorophyll, and is beneficial in alkaline soil where sulphate helps acidify the soil. In good, friable soil and normal conditions, this treatment, while not harmful, is not necessary.
Excellent, quick results are achieved after fertilising with LAN (limestone ammonium nitrate) at a rate of between 50 to 80g per mature bush.
Problems arising from extreme weather conditions
Much of this has to do with what is commonly known as sap flow, actually it is the circulation of water and dissolved minerals (nutrients) flowing upwards from the roots into the tip of every leaf. The strongest flow is to the highest points to the leaves which are exposed to the sun most and where evaporation is the strongest. These vertical bundles of tubes known as the xylem are on the outside of the wood. Part of the water is lost due to evaporation and the other part combines minerals and sugars created by photosynthesis to form amino acids. This sap is thicker than water and cannot be lost through evaporation. It is transported via the phloem, a porous tube, downwards and sideways to all growing parts of the plants and to the roots. The phloem is on the outside just beneath the bark. The important part one needs to understand is that the leaf acts as a sort of valve. Without leaves there is no downflow and a stagnation of the water that was pushed upwards. The sun heats up stagnant water in xylem to the degree of killing cells and tissue causing what is called sunburn.
Although sunburn is not a disease in itself, it causes a secondary disease known as stem canker or coniothirium. This is caused by bacteria and no remedial sprays are available. It causes ‘die-back’ or rather ‘die–up’ of part of the rose bush, or even the entire bush. A secondary fungus and bacteria settle in dead tissue almost immediately, and soon attack the surrounding healthy tissue as well. Black to purplish blotches become visible on the stem. The xylem capillaries through which sap is transported are blocked or severed at this point and, once the dead tissue has encircled the stem, the above section shrivels and dies.
Sunburn occurs on the lower parts of the stems, usually on the curvature just above soil level. It can happen in early August, when the stems are still bare after pruning, and the sun rapidly warms frozen tissue after a frosty night. In regions that expect late frost, it is advisable to spray after pruning with a solution of 1 part lime sulphur to 5 parts water, or use limewash. The residual whiteness reflects sunlight.
Sunburn occurs more often and with more devastating results in the heat of summer, when the lower parts of the plant are not shaded by a canopy of leaves and when water in the surrounding soil is allowed to heat up because of insufficient mulch or cover. Sudden leaf drop (caused by black spot, spider mites, hail etc.) can slow down the sap flow so much that normal cooling processes – liquid moving through the capillaries – does not take place.
If blotches are apparent but have not spread too much upwards (they never move downwards), it is still possible to retain the affected stems by sealing them with Steriseal. The stems will put on new growth and, with the sap flow restored, callus growth around the edges of the infected areas will prevent the disease from spreading. Such branches can be removed at a later stage during routine pruning, by which stage new basal stems will have taken over.
Another, often devastating problem that is caused indirectly by a slowing down and concentration of sap in the phloem is a proliferation of red spider and scale insects.
A cold snap in September can burn the tips of soft, new shoots. This happens almost annually in the coldest regions of the country. If the cold is not too severe, it may turn out to be beneficial – by performing a good pinching of shoots, after which rose bushes will sprout very quickly again, and with renewed vigour. However, if the frost is very severe, symptoms of sunburn (described above) may occur.
Climatic conditions and weather patterns play a major role in determining the well being of roses. Preemptive spraying can lessen the impact of adverse weather conditions.