Mr Fluttergrub’s garden – July

Aug 1, 2023

Plums: Promise and Peril

It’s mid-July and the fruits of the earliest cropping plum on the plot have turned pale purple. There’s no ‘give’ yet, that slight sponginess when the fruits are gently prodded to test for ripeness, so we’re probably still a week or so off the first picking. But at least we have the promise of plums. This makes a change as the last few seasons have been almost ‘plumless’. Various difficulties had me wondering whether there’d ever be a worthwhile crop of plums again!

Never count your plums before they’re picked. While Early Rivers, that’s the variety with the near ripe fruit, will almost certainly give a crop, the proof is in the eating – or in this case cooking – Early Rivers is mainly used as a culinary plum. There’s also a heavy fruit set on the tree that marks the end of the plum season, the September cropping Marjorie’s Seedling. Miraculously, there’s even some fruit set on the gages, including the notoriously shy fruiting but delectable tasting Old Greengage. The only trees to let the show down are Victoria and Czar out on the field orchard which, being more exposed, was hit by poor late March and early April weather.

Early Rivers culinary plums, one of the earliest plums to crop, nearing full ripeness

Several problems could yet mar the harvest, even that from the almost ready Early Rivers. These include the fungus brown rot which is worse in wet conditions, and today it has been raining heavily. Then there are the wasps that have been buzzing menacingly on the gooseberries and blackcurrants near the Early Rivers in the sunny breaks between the downpours. I’ve already seen a wasp’s posterior protruding from a hole in a damaged plum. There is however, an even greater peril, one that’s all too familiar to me and, I expect, many others who grow plums organically.

Here’s a scenario those with plum trees may be familiar with. You’ve waited all summer for the plums to be ready and now the first fruits are dripping off the tree. The plums have turned colour and taken on that enticing, ‘eat me’ bloom. You pick one of the juiciest looking and take a bite but instead of the sweet, rich, flavour anticipated your taste buds are hit by an unpleasant, acrid, tang. On looking at the bitten fruit you notice that much of the centre around the stone is mushy and peppered with dark brown areas and specks. If you’re very unlucky you might also notice a small pinkish caterpillar with a brown head!

The caterpillar, if you’ve had the double misfortune of encountering along with the damage its activities cause, is that of the plum moth (Latin name Grapholita Funebrana). With a similar life cycle to the codling moth that plagues apple trees, adult plum moths are on the wing through the Summer but it’s mainly in June and July that they lay eggs on developing plums. Usually laid singly on a fruitlet these hatch in about ten days, the tiny caterpillars burrowing into the developing plum to feed on the flesh. It takes a few weeks for the caterpillars to mature after which they exit through a small hole to form silken cocoons in bark crevices or similar hiding places. Here they overwinter before pupating in late Spring as a new generation of moths.

Plum moth is not an easy pest to counter organically. An option often suggested is pheromone traps. These are open sided triangular shaped boxes at the bottom of which is a sticky sheet. On this sits a pheromone pellet. This emits a synthetic copy of the scent of a female plum moth. The aim is to lure male moths so that they adhere to the sticky sheet and are unable to mate with females. At least that’s the theory. It’s important to note that non-organic commercial growers only use these traps as an ‘indicator’, not a control. In my experience traps have minimal impact. Luring in male moths means the moths just have an even bigger party!

Unripe fruit of the ‘Prince of Plums’, Old Greengage – should ripen in late August

A better alternative might be an approach that disrupts the life cycle of the moth. Unlike codling moth affected apples, plums attacked by moth caterpillars show little external sign of damage. However, affected fruits do tend to ripen prematurely, sometimes even dropping from the tree. Fallen plums and the first to mature should therefore be regarded with suspicion. Eat the first plums that ripen with caution. It’s better to take an exploratory nibble than a big bite! Do not throw infested fruits on the ground in disgust. Dispose of them, and any fallen fruits, deep in a hot compost heap so that the caterpillar, if still present, cannot develop into a moth to hibernate and lay eggs next season.

If past seasons are anything to go by, there’s bound to be some plum moths this year. But in previous years of heavier crops infestation levels by the pest seem not to have increased in proportion. I’m hopeful this’ll be the case and that Grapholita Funebrana, the peril of plums, won’t yet spoil a crop showing so much promise.

A big thanks yet again to Mr Fluttergrub for sharing his knowledge with us.

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