Wednesday, 20 August 2014

ROSE PEST AND DISEASES




Rose gardens are so popular but I often feel disappointed when I visit them due to the amount of pest and disease damage they often exhibit and the lack of decent flowers.  This is often due to inadequate pest and disease control and poor pruning they receive. However with a little care and regular inspection you can control the majority of pest and diseases that can affect your plants and ensure a great flower display. 

Roses can suffer from numerous diseases such as rose blindness, rust, black spot, mildew and rose sickness. Pest include aphids, brown scale and rose leaf rolling sawfly.  

Rose blindness

Roses that fail to produce flowers suffer from rose blindness.  Flower shoots develop normally but fail to form a terminal bud, or the bud will be empty or dry.  Leaf and stem growth appears healthy.  The cause is unknown but likely to be linked to weather conditions. Prevent rose blindness by growing roses in open, sunny positions. Cut back affected shoots by half to a strong bud. Feed affected plants with a rose fertiliser and add a mulch. In the winter prune out some of the old wood to encourage new shoots. 

Rust

Rust is a disease caused by the parasitic fungus Phragmidium tuberculatum.   In spring yellow spots appear on the upper side of the leaves, whilst dusty orange pustules appear on the lower surface. Prune out infected areas and fallen leaves as soon as they are detected to prevent the spread of spores.  Alternatively fungicides can be used. Most modern roses should be resistant to rust. 


Black spot

Rose black spot is is caused by a fungus Diplocarpon rosae.  It that causes black or purple spots on the leaves which yellow and drop early. The fungus adapts rapidly and so resistance to black spot is short lived. Popular garden varieties of hybrid teas, floribundas, climbers and patio types are usually susceptible.

You can collect and burn fallen leaves in autumn  However it is very likely that chemical control will have to be used and a suitable fungicide is often required. .

Mildew
Mildew is a white, powdery fungal growth Podosphaera pannosa. It affects the shoots and leaves. Flower buds may fail to open and shoots may become thickened. High humidity, dry soils or poor air circulation can make mildew worse. Water roses well in during dry spells and apply a mulch to prevent moisture loss.  Feed regularly to ensure vigour but avoid too much nitrogen as this encourages soft growth which is likely to be affected.  Prune out badly affected shoots and discard. Alternatively use a fungicide to control mildew, which has the benefit of also controlling black spot and rust.

Rose sickness

Roses are particularly prone to die back when planted in beds where roses have previously been planted. Roses planted in rose beds often suffer from lack of growth and vigour. This can occur just a few months after planting. Remove the rose, shake off the soil and plant it in an area of the garden where roses have not grown before and it will often recover. It is best to replant roses in new soil where roses have not grown before but if this is not possible then you will have to provide the rose with new soil around its roots.  Dig a hole 2 foot in diameter by 1 foot deep and replace with fresh soil from another part of the garden. Incorporate a nitrogen rich fertiliser into the planting hole.

Aphids

Aphids are also known as greenfly or black fly. They are sap sucking insects that can cause damage to the new growth and secrete a sticky substance called honeydew, which can cause the growth of sooty moulds. Some aphids can also transit virus's.

There are more than 500 species in Britain.   They are most active spring to summer when you may notice aphids around the new shoot tip, flower buds and under new leaves.  This can lead to curled and distorted leaves.

Aphids have many natural enemies, including ladybirds, hover fly larvae, lacewing larvae and parasitic wasps.  Crush aphids with your fingers on small scale infestations or spray with a chemical or organic pesticides.  Do not spray plants in flower due to the risk of affecting pollinating insects.

Brown scale

Brown scale is a sap sucking insect that lives on bark.  Deal with heavy infestations during summer when newly hatched scales are present by spraying with an systematic insecticide.

Rose leaf rolling sawfly


This insect lays its eggs in rose leaflets and secretes chemicals that induces the leaf rolling. The caterpillar-like larvae emerge and feed within the rolled leaflets.  Pick off infected leaves if lightly affected.  Chemical control is difficult as insecticides often do not reach the larvae in the rolled leaves and several applications are required to kill the adult.  Plants in flower cannot be treated as pollinating insects may be affected.

For related articles click onto:
Aphids
Can you keep bees in your garden?
Drainage
Feeding plants
How do I attract bees into my garden?
How to make compost
How to propagate using division
How to propagate from seed
Manuring
Soil structure
Watering plants
What is green manure?
Thinning and transplanting
Vegetable crop rotation
Weeding



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