Plum Rust - what to do about it

A question popped up recently about Plum Rust on a young tree: it was a question in four parts, so let's go through them one by one.

1) Firstly, what exactly is "Plum Rust"?

"Rust" is a catch-all term for a number of fungal infections, which show themselves in the form of brown dusty material on the leaves. Many plants suffer from various types of rust - trees, shrubs, herbaceous, bulbs, veg, you name it -  and the infections are usually specific to a particular genus, hence the names Plum Rust, Pear Rust, Hollyhock Rust, etc.

The symptoms usually appear towards the end of summer, and into winter, and take the form of brown dots, brown-orange patches, pustules, all sorts of lovely things, on the leaves: usually on the undersides of the leaves at first.

Often, the whole tree is affected, sometimes it starts in one place and spread across: sometimes only one or two branches are affected.

As it's a fungal infection, it is spread by spores, which float gently on the soft, winnowing winds of summer and autumn (said she, with her hair blown back off her face as though by a force 9 gale) so there is pretty much no way to prevent infection altogether.

The key seems to be in "managing" it: reducing the situations in which fungus thrives is a good start, and that means pruning to get the "open, goblet shape" so beloved of the RHS, who never seem to realise that out here in the real world, trees simply don't grow in the neat, tidy way, shown in their pruning illustrations.... anyway, prune out branches which cross each other, and which point inwards, in order to allow as much air flow as possible.

Yes, that does mean that you are reducing the amount of fruit it might bear next year, but it's better to have slightly less, good fruit, than to have a tree full of fungus.

The pruning will help to prevent pockets of still, moist air where a fungus could thrive.

Hygiene is also important, when dealing with fungus: when you have it, clear up all fallen leaves and burn them: don't compost them, or put them into leaf mold pens. (If you are not allowed to have bonfires, Also, check within the tree for any dead branches, and remove them. If the infestation is really bad, it's worth carefully picking off what leaves remain, as they are not going to be doing much in the way of photosynthesising, so they might as well join the others on the bonfire heap. 

2)  Having said "prune it into an open goblet shape, and cut out any dead wood", there is also the issue of silver leaf disease.

Traditionally, the books will say "only prune stone fruits in mid summer, to avoid silver leaf disease" but that's a bit of a simplification, for the benefit of non-gardeners. In real life, we sometimes have to prune at the "wrong" time of year, and that's when it's important to know the real reason behind these gardening rules.

In the case of silver leaf, it's wet weather that spreads the spores, so it's ok for professionals to prune stone fruits (ie plums, apricots, anything with one solid stone inside, rather than lots of pips) in winter, as long as it's during a dry spell. Which is not always easy to forecast, admittedly! So it's always safer to prune stone fruits in summer if you possibly can.

3)  The third part of the question concerned the fruit:  the owners had said "they were advised not to eat the plums, in the first year the tree bore fruit. Is that right?"

The usual advice is to pick off the fruit as it starts to form, for the first year or two of fruiting, so that the young tree puts its energy into rooting, not fruiting.

If the owners didn't do this, then they might as well eat the fruit, as waste it... unless the rust has also extended to the fruit, in which case they would be advised not to eat the fruit. 

4) Finally, we have a very reasonable question - Is there any any remedial natural spray besides the garden hygiene to apply in autumn?

Traditionally, spraying against rust infections is done at blossom time, in spring/early summer, and it involves chemical fungicides. In simple terms, no, there aren't any  "natural" sprays which will control it, especially this late in the year, by which time the damage is done.

In many cases, no control is actually necessary: the trees are about to lose their leaves anyway, and the RHS do say "many rusts of trees develop too late in the summer to have a significant effect on  vigour, even though the whole tree may appear yellow or orange in late summer due to the huge number of rust pustules on the leaves."

So don't waste time with garlic sprays, or banana peel sprays (*laughs*), just be sure to rake up every single rusty leaf which falls, and burn them, along with any other debris below the tree.
 

 

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