Wednesday, July 19, 2017

Controlling curly leaf

Curly leaf can have a devastating impact on stone fruit trees in spring. This blog post discusses methods for preventing curly leaf, looks at what to do once you have it, and discusses organic ways of dealing with this garden pest.


What is curly leaf?

Curly leaf, or more officially peach leaf curl (Taphrina deformans), is a fungal disease of peaches, almonds, nectarines and occasionally apricots and ornamental prunus, which causes severely distorted leaves, making them fall prematurely.

curly leaf in action - photo by Ontario Ministry of Agriculture

Curly Leaf operates as follows 
  • Curly leaf overwinters within leaf and flower buds, and under loose bark. 
  • When buds open, the fungus penetrates the leaf tissue and flowers. 
  • Signs of curly leaf usually appear within two weeks of leaf emergence. Reddish areas on developing leaves are followed by massive swelling and distortion. 
  • Curly leaf is worse if the weather is cool and wet during leaf emergence. Infections are greater following rain, as spores are washed into buds. 
  • It now produces a bloom of spores on infected leaves. These spores lodge underneath loose bark, around buds and in other protected areas, and, if not killed, will repeat the process next spring. 
  • Infected flowers and leaves will die and fall off. Fruit that manages to develop can also be infected, causing reddish pimples followed by fruit drop. 
  • Disease development stops when warmer weather (over 20 degrees C) arrives. The tree may now produce a second flush of leaves to replace the fallen, infected leaves. These new leaves will not be re-infected by the fungus. However, the tree may have been weakened a lot, and therefore produce less or no fruit this summer. 
As with all food garden pests, knowing curly leaf’s behaviour is the key to combating it effectively.


General fruit tree maintenance and hygiene 

When the tree has gone dormant (May – July) and before new buds begin to develop do the following:

  • Remove all of last season’s leaves, left-over fruits, loose bark and other opportunities for spores to hide from the tree. 
  • Remove last season’s debris and mulch from the ground underneath tree. 
  • Improve circulation by pruning the tree so it has an open vase shape. 
  • Put all these materials in the rubbish bin, NOT on the compost heap. They can contain curly leaf spores, so you no longer want them in your garden. 
  • Fertilise your tree (with Complete Organic Fertiliser ideally, but blood and bone on its own is also good), rake this in, and water if the ground is dry. 
  • Now apply one or more of the targeted control methods described below. Timing is really important with this. The methods described below need to be applied in the period of dormancy to just before buds open (in Tasmania generally June – July). Once buds are opening or are open you are simply too late. The control methods described below will have no effect, or worse, they will damage leaves. 
From when buds open until end of season always make sure your tree is well fed and irrigated. This is especially important if it was weakened by curly leaf in spring.


Control Method 1: Lime-sulfur fungicides 

  • Low toxicity. Often claimed to be ‘organic’, but that depends on your definition. 
  • Sold as a ready to use spray 
  • Some claim it is not as effective as the copper spray discussed below. 
  • Spray on a day when there is no wind or rain. 
  • Spray before buds have opened. 
  • Do not spray after buds have opened. 
  • Prepare spray according to instructions on bottle or packaging 
  • Wear a face mask, adhere to safety directions, and wash hands and clothes afterwards 
  • Spray the whole tree starting at the top and all the way down to the bottom 
  • For best result spray a second time around a week later. 

Control Method 2: Copper-based fungicides 

  • Low toxicity. Often claimed to be ‘organic’, but that depends on your definition. 
  • Sold as a ready to use spray 
  • Long-term frequent use can lead to a build up of copper in your soil 
  • Spray on a day when there is no wind or rain. 
  • Spray before buds have opened. 
  • Do not spray after buds have opened. 
  • Prepare spray according to instructions on bottle or packaging 
  • Wear a face mask, adhere to safety directions, and wash hands and clothes afterwards 
  • Spray the whole tree starting at the top and all the way down to the bottom 
  • For best result spray a second time around a week later. 

Bordeaux is an old-fashioned copper-based fungicide that can be cheaply made in whatever quantity you like at home. Here is how you make it:

  1. Just before you are going to spray, dissolve 100 grams of builders’ (hydrated) lime in half a standard bucket of water. 
  2. Dissolve 100 grams of copper sulphate (buy at garden centres) in a separate half bucket of water. 
  3. Keep the lime mixture agitated to prevent settling and pour it steadily into the half bucket of dissolved copper sulphate. 
  4. This is Bordeaux mixture. It is at its most effective when freshly mixed. Use immediately or within a couple of days.
 
  5. Spray the whole tree starting at the top and all the way down to the bottom 
Bordeaux is used in vineyards, fruit farms and gardens to prevent infestations of fungi like downy mildew, powdery mildew and curly leaf.

All dot points above for copper-based fungicides apply, except ‘sold as a ready to use spray’.


Control Method 3: this method is completely organic 

On the page devoted to curly leaf on the web site of the British Royal Horticultural Society I found 


I concluded that the fungicides mentioned above are not for sale in Great Britain because they are deemed to be too toxic. I believe this to be overly cautious, but it is great to see that it lead to rigorous experimentation by the Royal Horticultural Society at its Wisley Model Fruit Garden (Surrey, south of London) to find ways to avoid Curly Leaf in an organic way.

The totally reliable method they came up with was derived from an observation made earlier in this blog post:

Curly leaf is worse if the weather is cool and wet during leaf emergence. Infections are greater following rain, as spores are washed into buds.

The incredibly simple truth is that, if you make sure that your stone fruit tree is not rained on at all between when it drops leaves and when buds open (roughly May to September in Tasmanian conditions), you will not get curly leaf.

On the Royal Horticultural Society web page it says (note: dates are for Northern Hemisphere):

Where peaches are grown trained against a fence or wall, a rain shelter of plastic sheeting is very effective at preventing infection. It should cover the top of the tree and the front to within 30cm (1ft) of the ground, but with the ends open to allow access for pollinating insects. It should be erected after leaf fall in November and kept in place until mid-May. Keeping the emerging shoots dry in this way prevents infection and also gives useful frost protection. It is used successfully here every year. A diagram of how to make the lean-to is available as a download.
Yes, a photo and a diagram of their rain shelter can be found here:
https://www.rhs.org.uk/advice/profile?pid=232

Thank you Royal Horticultural Society. What a brilliant solution!

Of course this won’t work for high wide large trees, but from what has been written on this blog about fruit trees, you may understand that they are not great for many reasons anyway. This totally organic efficient solution will work well with espaliered or dwarf stone fruit trees.


If you find your tree has curly leaf

Most infestations happen because trees were not sprayed, or sprayed too late, or areas on trees were left un-sprayed. So what do you do if leaves begin to show the signs of curly leaf?

Do NOT use the fungicides described above once flowers and leaves have budded. You are simply too late AND your sprays may damage young growth, making the problem worse.

Many people claim that cutting affected leaves from the tree and collecting affected fallen leaves is the way to go. If you do this, put them in the rubbish bin, not in your compost heap.

However, researchers found that this has little or no effect on fungal spore count. If it makes you feel better, by all means go ahead, but don’t remove all leaves if the infestation is bad. The tree may die.

If disease problems are severe, maintain tree health and vigour by thinning more fruit than normal, watering regularly (avoiding wetting the leaves if possible) and feeding organic fertilisers high in nitrogen in order to help the tree to recover.


Fruit tree selection and planting

When choosing a new stone fruit tree at a nursery look for
  1. Dwarf varieties - there are now great-tasting dwarf varieties that make maintenance, protection against pests, and picking a lot easier 
  2. Trees that lend themselves to be espaliered 
  3. Varieties that have some resistance to curly leaf 
Agriculture Victoria, on a Victorian State Government site web site, says:

Some cultivars show resistance to leaf curl, but apparent resistance observed in the field may be due to different times of bud movement which may avoid favourable conditions in one season, only to become infected in another season following different weather conditions.
(see http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/plant-diseases/fruit-and-nuts/stone-fruit-diseases/peach-leaf-curl - last update Aug 2010)

Various internet sources mention varieties that are resistant to curly leaf, but I don’t know whether these varieties are available in Australia. It is clear that breeders are trying to create varieties that are resistant to curly leaf, so keep your eye out for them.

Choose a spot for your new stone fruit tree that is sunny, warm and handy for protection by plastic sheeting (a fence or wall?) in the period of May to September.

Install drip lines, so you never have to irrigate your tree from above (see http://foodgardengroup.blogspot.com.au/2015/02/improving-your-irrigation.html)


At the end of this blog post

I like to thank two members of the Food Garden Group, Margaret W (entomologist) and Max K (fruit tree professional), for reading a draft of this post and providing additional info and feedback!

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