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December 2012

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Maple die-back, hellebore black death and rhizoctonia!

 

An important strategy for control of pests and diseases is to have healthy plants. Plants growing well in suitable conditions can often recover better and suffer less from disease attacks, and this was never more evident than the summer gone. It is important in the first place to procure only healthy plants. This is easier said than done sometimes as some diseases can be masked and only appear after several seasons growing. Good examples of these are pathogens affecting the lovely Japanese maples group and Cornus controversa ‘Variegata'. Affected acer plants suffer dieback of whole branches and may not come out of winter dormancy. Cornus might shed whole leafy branches especially from late summer. Prompt removal and destruction is advisable.

 

Black death disease of hellebores is caused by virus diseases and can kill affected plants. Blackening of veins and surrounding tissues with mottling are the symptoms to look out for and prompt removal and destruction is essential. These symptoms are most frequently seen on the foliage from April to June. Be aware that this disease can be spread from plant to plant when old leaf removal is carried out earlier in the year. Secateurs should be sterilised after each plant.

 

Soil type can have a strong influence on disease, particularly where rainfall has been exceptionally high. Lavender suffers from serious leaf spot when grown in heavy soil. The various species of outdoor cyclamen can have high casualties where planted in heavy or poorly structured soils and die from root rot or fusarium fungus, especially Cyclamen hederifolium. If these are planted under trees or shrubs, which take up much of the excess moisture, few problems arise. Place the corms two or three centimetres under the soil surface. Plenty of organic matter and copious amounts of lime help enormously in providing the loose, open, friable, free-draining soil required.

 

Species of orchids also suffer badly in heavy or poorly structured soils due to slug attacks and root-rot fungi, such as pythium or rhizoctonia. Planting orchids in open, raised sand beds has greatly reduced this problem as slugs and fungi find these areas inhospitable. Some alpines are difficult to grow, such as Anthyllis, Erodium and Campanula pulla, and will only thrive in very sandy areas with some organic matter mixed in. I have used companion planting also to reduce bulb and corm losses, and this is quite effective as the companion plants take up excessive moisture -much the same way as underplanting of trees and shrubs. Many of the summer-flowering bulbs, such as allium and ornithogalum, suffered losses from the exceptional heavy rainfall.

 

Foliar diseases were very challenging last summer. Fungal diseases, such as downy mildew of impatiens, were widespread under the continuously wet conditions. Scab was common on apple trees, and leaf spot on photinia and holly, while tar spot of sycamore trees was very severe, inducing early leaf fall. Black spot on roses was a major problem but there are plenty of cultivars with adequate black spot resistance. Rosa chinensis ‘Little White Pet' is susceptible to black spot but the bushes always recover. Box blight has been very severe especially with a large number of plants growing closely together. Rusts have likewise been problematic and there was a lot of potato blight. All in all, it was a year when fungi were the winners!

 

 

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