Healthy Roses - Part 2

Michael Marriott

Part 1 of this article described how to grow healthy roses by choosing varieties that are disease resistant, using judicious planting schemes and by feeding well with mulches and other nutrients.

Roses love a good soaking. We can learn a great deal from rose gardeners of North America where their excessively dry hot summers encourage them to water on a regular basis. But with our climate in the UK, we think that our normal rainfall should be enough. In fact an occasional soaking helps a great deal in making roses healthier while promoting both better quality and repeat flowering blooms. Powdery mildew which is quite common on climbers is often due to dryness at the roots, especially when these roses are planted against a wall on the eastem or southern face.
    Spring is the crucial time to water because then plants are growing rapidly and are most likely to suffer from stress if the moisture is insufficient and stress can make a plant more susceptible to infection. So it is important to soak the soil well so that it reaches the deepest roots. Frequent but minimal watering can do more harm than good as it encourages shallow roots which can easily become dry and then shrivel once the watering stops.
    Although watering the leaves can help control powdery mildew, it can also encourage blackspot. On balance it is probably better not to wet the leaves, but if you do, water on a warm day in the morning so that the leaves will dry out quickly; blackspot spores should then have little chance of germinating and becoming established.

Removing diseased leaves
    Young shoots that are particularly badly affected with powdery mildew can be cut off and disposed of, but be careful not to spread the spores about. Similarly shoots covered with aphids can be cut off; an alternative and very effective method (but rather less pleasant) is to squash the offending pests between finger and thumb. Fallen leaves infected with blackspot and rust should be disposed of during and at the end of the season - but not on the compost heap. Some picking off of infected leaves from the rose bush itself can be done, but it is probably best not to be too over enthusiastic as it can deprive the plant of vital leaf area. Plant physiologists tell us that when a leaf is about 30 days old it is no longer photosynthetically active and so removing the older, disease ridden leaves should not be a problem to the plant. The mulch applied in early spring will bury any blackspot spores that might still have been alive on the soil surface.

Fertilisers
    In the first part of this article (Autumn 1999), I discussed the importance of adding fertiliser to promote a strongly growing bush so making it much more resistant to disease. Since then several products have been introduced onto the market that claim to feed roses (and plants in general) while at the same time encouraging disease resistance. All contain beneficial bacteria and fungi. Nurture is available in the UK (from ALS only by mail order, tele 01952 641949). Trials in the USA have shown it to be very effective. Restore is not a fertiliser but instead introduces bacteria into the soil breaking it down to release nutrients which are then available to the plant; (sold in the UK by Biotal it is available in garden centres). These products sound like a good idea and are probably worth trying.

Replant disease
    Replant disease can be a problem for roses. It seems to appear when a newly planted rose - replacing one that has been in the same spot for many years - has weak growth and becomes diseased. If one rose must replace another in the same position then it is best to change the soil down to a depth of about 30x45cm (12-18in) x 45em (18in) wide, adding a generous quantity of well rotted manure.
    The benefits of Armillatox for replant disease have been talked about but unfortunately no proper trials have been carried out to determine its real value. The bacterial and fungal-rich fertilisers are also claimed to work and would be worth trying. But I am quite confident that if the soil has been well treated with annual mulches it will be in good condition and replant disease is then unlikely to be a problem. A good deep digging and the addition of plenty of well rotted manure is likely to be very effective. Replant disease is much more likely to occur in rose borders of long standing, where the soil has been trampled down over the years and the soil level reduced by several inches.

Spraying
    Perhaps everything has been done to encourage healthy growth, yet diseases or pests still remain. Or there may be a particular variety that is susceptible to disease but which one still wants to grow. What can de done? Spraying can be the best course of action. Since there is a long tradition of focusing solely on pests and diseases do ensure that it is worthwhile to spray. It is a matter of knowing the varieties; some will fight off disease while others will get worse; if the latter, swift remedial action is essential.
    Once the roses start growing and the early leaves are fully expanded this is the time to start spraying. Indeed the most important spray of the whole season is the first and the timing of this will depend on where one gardens. For instance in the south of England this is likely to be early to late March depending on when spring comes. Further north the first spray may be a little later. Likewise in other countries the timing of spring and early rose growth will depend on the weather. This first spray will help to get rid of the early infections from over wintering spores. But beware of frost the night after spraying as this can scorch the leaves very badly. This first spray should be followed by another about two weeks later. Two early applications in the spring followed by another later on should reduce or obviate the need for regular sprays throughout the summer.
    Nimrod T is the best spray against blackspot and powdery mildew; it has some effect against rust. The best spray against rust - which tends not to appear until rather later in the season - is Systhane, although an early application is certainly beneficial. Systhane also has some effect on blackspot and mildew. Some rosarians have used Armillatox as a winter spray with apparently good results.
    Aphids are often very seasonal and so it is much better to apply a specific spray against them as and when they are a serious problem. If the weather is fairly cold then apply a general insecticide but if the daytime temperatures are 15º C (60º F) or above then it is much better to use a specific aphicide, one which contains Pirimicarb. This will only kill the aphids and none of the other hisects, be they beneficial or otherwise. By regularly spraying with products which combine the fungicide and aphicide (eg Roseclear), aphid populations resistant to insecticides will be encouraged. These are becoming more and more common in agriculture and are remarkably difficult to kill. Therefore such sprays should not be over used.

Rules for spraying
Here are a few basic rules when spraying:-
Follow the instructions on the label very carefully.
Wear appropriate protective clothing.
Do not mix sprays together unless there are clear instructions that it is safe to do so.
Do not apply insecticide more frequently than is recommended as it can do more harm than good.
Wash the sprayer out very thoroughly after using it.
Never use a sprayer that has been used for Glyphosate (Roundup) as roses are extremely susceptible to it.
Never apply a spray if the plant is stressed, for example due to dryness at the roots, or high temperature.
Most Rugosas are very resistant to disease but also sensitive to sprays so do not spray them.

Organic methods
    Unfortunately the three main diseases do not particularly lend themselves to be controlled by organic means other than those described under good cultivation. Specific organic sprays you may wish to try are sodium bicarbonate, 2-10g/1 of water, plus soft soap as a wetting agent for powdery mildew. For blackspot, Equisetum tea is meant to help strengthen resistance, as is seaweed extract. It is said by some that garlic and onions help to prevent blackspot. There is unfortunately no specific advice for rust other than good cultivation. It is well known that ladybirds eat aphids and apparently, so do earwigs, bats and various insect larvae including hoverflies and lacewings; these can be encouraged by planting suitable host plants. Insecticidal soaps and derris also work but only by contact. Personally I prefer the finger and thumb method.
    I hope all these words do not make the job of growing healthy roses sound too complicated. I am passionately keen that gardeners should grow more roses since they give so much pleasure and most probably with a little less effort even than I have suggested here.
This article appeared in the Spring 2000 issue


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