Horticulture notes June 2007

Growers may come across a yellow or brown edge die back on leaves of laurel hedging plants. This is found on plants on the nursery and does not occur normally in gardens. Other symptoms are brown spots on leaves with the spot surrounded by a yellow halo. The areas fall out giving a distinctive 'shot hole' appearance. It is called this because when the brown areas are pushed, they fall out and usually give the foliage the appearance of being peppered with gun shot. A bacterial canker organism causes these symptoms.

Laurels are very prone to this problem and it is easily spread with overhead irrigation.Growers should try to reduce the frequency of overhead irrigation where possible.

Trimming and pruning of plants should be done during dry weather to avoid spreading the bacteria. In some cases it may be necessary to apply sprays of a copper based fungicide to help control this problem.

With few approved systemic insecticides available, crop monitoring is vital to ensure that spray timing is effective. Pest predictions alert growers to the need to examine crops and take action where pests are found.

Predictions are that pests will occur earlier and in high numbers. Peach potato aphid is widespread and the first cabbage aphids and carrot willow aphids are visible in some crops.  

Aphid populations build up rapidly in warm conditions. Carrot flies are laying eggs and carrot and parsnips crops past cotyledon stage need protection. The first generation is expected to end in early June.

Flea beetles are causing damage in some brassicas. These beetles eat holes in leaves, often attacking once the cotyledons are visible. At this stage they can cause serious damage or even kill plants.  They feed for just for a few weeks before hibernating.  

The first cabbage white butterflies have been sighted so egg laying can be expected shortly. The caterpillars will then hatch in approximately 7-10 days.

Chemical control of all pests relies on the appropriate selection and effective timing of insecticides. It is good policy not to rely on one insecticide alone - alternate chemicals with different modes of action.

For non-systemic insecticides use only when pests are present, apply at recommended rates when conditions are favourable.

A fine start to the season, but May weather encouraged apple scab.  Almost continuous scab infection periods were recorded between 6 and 12 May at AFBI, Loughgall, and again from 14 to 17 of May.

You may have had difficulty maintaining spray cover. Where changeable weather is forecast it is best to shorten the spray interval rather than risk a delay beyond the period of protection.  

Remember too, the curative properties of eradicant scab fungicides only work if applied within the ‘kickback’ period following spore germination.  This period is typically 72 hours (or less) after the infection period.

In 2006, June’s dry, hot weather led to outbreaks of apple powdery mildew (Podosphaera leucotricha) in some orchards.  This fungal disease is present at low levels in any orchard but your scab programme is usually enough to keep it in check.  

If hot, dry weather returns, you must check on levels of mildew.  A serious secondary infection can arise within five to seven days during high summer, resulting in dull, discoloured leaves, poor fruit growth and a more established disease next season.  

Approved fungicides to tackle powdery mildew infection include those with DMI  (for example, penconazole), as well as bupirimate (Nimrod), dinocap (Karathane liquid) or the combination of boscalid + pyraclostrobin in ‘Bellis’. It is essential to follow the directions on the label for handling, storage and use.  For further information, please contact your development adviser.