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Page 4

Garden Plant Disorders.

Pests and diseases of shrubs and trees.

           Trees and shrubs, like all living things are susceptible to disease. They need a good supply of light, water, carbon dioxide, and nutrients. If they lack any of these trees or shrubs could come under stress and then not have the reserves of energy for their defensive systems, and hence, become vulnerable to disease.
  • Symptoms:  The leaves display  cream  or yellow spots like a mosaic.  Serious infection can cause  the foliage to deform and the plant to wither.
  • Shrubs:  affected:  Roses and other  ornamental  species.
  • Treatment:   None  available. lf  the attack  is not serious, it is enough  to prune the affected parts. Otherwise,  dig up and burn the shrub.
  • Symptoms:  Leaves at the tips of shoots are pierced with minuscule holes;  eggs can  be seen  along  the principal  vein. These hatch into larvae that hollow a gallery  in the leaf, leaving a brownish  track. The adult insect  sometimes  eats the  edge  of the leaf.
  • Shrubs  and trees affected: Beech, buddleja,  hazelnut, pines  and others.
  • Treatment:  Spray  with bifenthrin  if the weevils are sufficient  to cause  problems.
  • Symptoms:  The leaves are devoured. Large cocoons  can be seen on the tips of branches.
  • Tree affected:  Oak.
  • Treatment:  Prune  out the cocoons before the caterpillars emerge. If they become active, spray with  Bacillus thuringiensis  or an appropriate approved  insecticide.
  • Typically, peach leaf curl attacks peach trees as well as ornamental plants,  such as camellias and poplars.
  • Psylloidea are sap-sucking insects found on ornamental  species,  most commonly  apple and pear trees.
Roses and dahlias are prone to this disease,  which is characterised by a white coating  on the leaves.
Red Spider Mite
  • Symptoms:  Leaves become dull, turn yellow with blotches, then fall. The undersides  are covered in a fine web and eggs.
 Shrubs  and trees affected: Most ornamental  species.
  • Treatment:  In a greenhouse, introduce  the natural  control predator  Phytotseiulus persimilis  as soon  as the pest is noticed.  In other  situations, spray with an approved  fatty acid or oil-based treatment.
  • Symptoms:  The leaves turn a yellow mottled colour.  The transparent  larvae of the insect can also  be seen.
  • Shrub affected: Rhododendron.
  • Treatment:  Avoid planting in warm, sunny  places.  Spray with a bifenthrin-based insecticide.
  • Symptoms:  Orange-yellow patches  develop  on the upper leaf surfaces,  with pustules  on the undersides  causing  the leaves to fall prematurely.
 Shrub affected:  Rose.
  • Treatment:  At the first sign of attack,  spray  the leaves  with an approved  fungicide.
Rose Rust
  •  Symptoms:  The leaves are perforated,  almost skeletonised,  leaving only the principal vein. The remaining foliage withers.
  • Shrub affected:  Rose.
  • Treatment:  Spray immediately  with an insecticide containing  pyrethrin  or derris. Repeat  several times.
  • Symptoms:  Numerous  black or brown spots  are visible on the underside of leaves and on the stalks. If pressed with a fingernail, a hard  covering can be felt.  Waxy or cottony
discharge  can  also be seen. A significant  presence of sooty mould is a characteristic sign.
  • Shrubs  and trees affected: Many species.
  • Treatment:  Due to the hard protective  coat, regular spraying with a fatty acid or oil-based insecticide may be necessary to control this pest.
  • Symptoms:  The leaves  take on a silvery tinge. As the infection spreads,  the tree withers  and dies back.
  • Shrubs  and trees affected: Members of the  Rosaceae family.
  • Treatment:   There is not a cure  for this disease.  Prune back affected growth to healthy white tissue  (infected wood has a dark stain) in the summer only. Treat the pruning wounds  by applying an approved  fungicide.
  • Symptoms:  This  pest leaves whitish specks on leaves, which form a silvery marbling.
  • Shrubs  and trees affected: Privet, palm,  gladiolus, ornamental  fruiting species.
  • Treatment:  In your greenhouse,  introduce  the predatory mite  Amblyseius,  or hang  up sticky  boards to trap the thrips. Plant Nicotiana sylvestris,  whose sticky  leaves act as a trap.
 This fungal  disease  attacks catalpa and Robinia  pseudoacacia, causing  the foliage to yellow and wither away.
  • Symptoms:  Irregular nicks occur around  the edges  of leaves and excreta can be seen on the leaf. Roots  are sometimes  devoured  by white larvae. leaving deep gashes.
  • Shrubs  and trees affected: Many plants  can be affected, including euonymus, hydrangea, rhododendron, yew, skimmia.
  • Treatment:  Drench  infested soil with  the natural control nematode Heterorhabditis megidis or an approved insecticide. Ring  planters with  grease bands  to trap adults before they lay eggs  in the compost.
  • Symptoms:  Variously coloured  caterpillars  devour the leaves and buds  of affected plants  and form woven, hanging,  silky nests.
  • Trees  affected:  Several plants including hawthorn, euonymus.  cotoneaster,  juniper.
  • Treatment:  Carefully  prune out and burn  the nests containing  the caterpillars. If the caterpillars  become active, spray with Bacillus thuringiensis  or an approved insecticide.
  • Symptoms:  Pale  green caterpillars  devour  the leaves, buds  and fruitlets. They leave a trail of silky threads.
  • Shrubs:  and trees affected: Carpinus,  rose.
  • Treatment:  Place grease bands  around  the trunks of affected  plants  and spray the caterpillars  with  Bacillus thuringiensis  or another approved  insecticide.
Woolly Adelgids
  • Symptoms:  The needles of affected conifers turn yellow. Those of the pine are covered with  white, waxy filaments.  On spruce,  galls appear  at the tips of branches.  Fluffy balls hang from the branches  of Douglas firs. On larch,  the insect  is covered  with a white  wool.
  •  Trees affected:  Conifers.
  • Treatment:   Adelgids  are not easy  to control,  but conifers are able to support  populations without any  ill effects.

 'Powdery mildew'
Powdery mildew is host specific, so different powdery mildew  infect different plants. The powdery mildew on your lilacs will not spread to your peaches or your roses, even though all powdery mildews favour the same conditions.
Powdery mildew overwinters in plant debris and begin producing spores in the spring. These spores are carried to your plants by wind, insects and splashing water.
To prevent Powdery Mildew
Dig in organic matter in autumn and apply a mulch during the spring to keep the roots moist.

Minimise humidity by thinning out your plants to avoid overcrowding and so give good air circulation.

 If you  have plants that are susceptible to powdery mildew, do not feed too much with nitrogen rich fertiliser. This tends to encourage sappy growth which is more susceptible to fungi.

Brown Lacewing on garden thistle
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