Journal for 26th March 2010It IS getting warmer:
After the much warmer weather of the last week, all plant life is growing away fast. The Primroses and Daffodils are a clear indicator of the rapid changes about us. Fruit trees are no exception, although the weather forecast suggests lower temperatures next week. Either way apple growers will be watching closely changes in fruit bud development as fungicide sprays to combat scab need to be applied as soon as young green tissue (a sign of what will be the first leaves) forces its way out of its protective winter coat. Of course scab will not necessarily infect the newly exposed tissue if the spores are not present and the weather is cool and dry.
Apple Scab (Venturia inaequalis):
Scab is the most economically challenging apple disease in the World. Most varieties are susceptible; very few are immune, however some like Discovery only suffer infection in extreme conditions while Gala is the most susceptible of the commercial varieties. Scab spores overwinter on leaf debris and other vegative debris (wood) on the orchard floor; if high levels of scab were present in the previous season it is highly probable scab will be present on the one year old wood on the tree. The next factor in the 'risk chain' is wet conditions and warm temperature. Leaves need to be wet for infection to take place and the release of spores increases as temperature rises. In the spring time overwintering scab inoculum; as Ascospores released by the rain from overwintering leaves on the orchard floor or as Conidia spores from tree sources infect young leaves and fruitlets. As the season develops growers assess the risk by careful inspection of the leaves for minute scab infections. This needs a magnifying glass to spot the first signs of leaf scab. Generally the process of infection is from old (overwintering) source onto young tender leaves and then, IF infection is high and weather wet and warm, conidia spores will infect the young fruitlets.
The essential elements of Best Practice require good orchard hygiene by reducing inoculm wherever possible. Measures such as maceration of leaf litter and removal of infected wood on the tree and Autumn fungicide sprays to eliminate/reduce overwintering inoculum and critically, by timing early fungicide sprays to protect the young tissue from infection.
If this regime is followed by vigilant inspection and regular early season sprays, it will be possible to stop scab sprays before mid summer. However if scab establishes itself on leaves and young fruit, the grower will fight a rearguard action all summer long, with money wasted on sprays and reduction of marketable fruit; altogether a potentially disastrous result. In dealing with the 'vigilant inspection' aspect, most growers will rely on regular orchard walking by using a trained advisor who will spend the entire season 'crop walking' and by virtue of his exposure to seasonal factors on many different farms and geographical locations will develop a keen eye for problems. One should add, there are always 'hot spots' (areas where a problem of disease or pest is more likely to prevail) where the experienced advisor will look first, thus influencing where and what to look for next!
Varieties flower at slightly different stages and equally some will reach bud burst ahead of others. In a mixed orchard scenario it is vital therefore to observe carefully for the first buds on the move! Often the pollinating variety will be first to reach bud burst and where a Malus (Crab apple) is used as a pollinator it will be the 'first to burst'
Observe the next two pictures; one of Malus and one of Gala both taken today (Friday 26th March) in the same orchard. While the Gala is ahead of the Cox and Bramley in adjacent orchards, the Malus is clearly well advanced and potentially at risk of scab infection. Once infected, it will in the right weather conditions, spread to the neighbouring Gala trees.
With very little in the armoury for use against scab, organic growers must depend heavily on a combination of; (1) Use of varieties with scab resistant genes; (2) Eradication of overwintering inoculum by removal of infection by leaf maceration, and tree management which allows good air movement allowing rapid drying of leaves and fruit after rain. Currently the only permissable spray for organic use against scab is Copper which is only effective as a protectant. To maximise the efficacy growers now follow a 'predictive model' rather than applying a routine (7 day) programme. Taking note of the local weather forecast is central to the model and the ability to apply the copper protectant in dry conditions before the onset of scab infection is critical.
Good weather is the best control agent; Keen gardeners will observe that scab infections are nil or minimal in years where the spring and early summer is predominately dry. Conventional apple & pear growers benefit greatly in dry conditions too. On some of the more susceptible organically grown varieties it can be a 'make or break' moment!
Well that's all for this week, but before signing off may I remind my readers of the Blossom Walk Competition!
A readers competition
The English Apple Man offers a blossom walk to the web visitor with the best description of 'Their favourite English Apple'
Describe in no more than 30 words the virtues of your favourite English Apple!
The prize will consist of a day with The English Apple Man and a walk through apple orchards in bloom and a pub lunch for two people. Location; Kent. Winner and companion to arrange their own transport to a mutually agreed destination in Kent.
Date to be mutually agreed; flexibility important; The EAM anticipates blossom around the end of 1st week in May.
Please send entries via the Contact page on the English Apple Man web site, by Friday 16th April. Winner will be notified by Friday 23rd April. 'St.Georges Day'
The English Apple Man