Apple scab

Adrian Baggaley provides some advice on controlling scab infection on apples and pears.

Everyone who grows a number of apple and pear varieties is sooner or later going to run into trouble with scab infection on leaves, shoots and fruit. The disease, caused by a fungus, is most obvious as black blemishes or blisters on the fruit, which as well as being disfiguring means that it will not keep. Grey brown patches also appear on the leaves and, in heavy infections, on the wood. It is more prevalent in wet regions and seasons and on poorly ventilated sites.

Certain varieties are definitely more susceptible than others and it will pay to avoid these, indeed, select ones that are, by experience or reputation, somewhat immune. Unfortunately those that are reputed to be immune can be just the opposite. Four of such apples in my own orchard, near Nottingham, are Pinova, Saturn, Red Devil and Falstaff.

Some years ago I read that Falstaff was classified as an ‘organic’ apple, a term applied to apples and possibly pears that are resistant or immune to scab and mildew. I very quickly found out that this was not true; in fact it took just six weeks to discover. Now, I view it as being in the top ten listing of scab factories. Several new Eastern European varieties are claimed to have good resistance to disease and I am currently testing: Otava, Pinova, Resi, Rubinella, Rubinstep and Topaz. Most are new plantings that have not yet cropped and it is too soon to see how they will perform, but an earlier planting of Pinova went down with scab at the first hurdle. The Czech apple Otava is well overdue to give me a crop, or any fruit at all. I assume this to be a tip bearer and when carrying out formative pruning I am in fact pruning away the fruit buds.

My own definition of a scab susceptible variety is one that succumbs year on year, compared with others. Orchards with no incidence of scab (how fortunate) will not tell you anything, however, as no comparisons can be drawn.

There are a number of long cherished varieties of apple and pear that claim scab resistance and a visit to Ampleforth Abbey’s orchards in Yorkshire with the Northern Fruit Group a few years prove very educational, since the orchard had been neglected for a couple of years. Out of the forty varieties grown all but three were infected with scab. Lord Lambourne, St Edmund’s Pippin and Lane’s Prince Albert had no scab at all and lived up to their reputations.

In my orchard, I have grown Lord Lambourne for around sixteen years and year on year the fruit is clean. Other varieties that remain clean over the years are: Charles Ross, Blenheim Orange, Discovery, Lord Derby, Winston, Holstein, Ingrid Marie, Red Alkmene and Pitmaston Pineapple. I also find a number that are usually clean: Alfriston, Egremont Russet, American Mother and Merton Beauty. Contrary to a recent Royal Horticultural Society article, the Reverend W. Wilks is definitely not scab resistant with me. In fact just the opposite and I lost my first tree to a severe scab infection.

So how can we tackle the problem of scab?The first step is to identify and remove the scab factories, the trees that are infected every year, although this is often a painful exercise. The pride of my life was the apple variety Crowngold, an excellent keeper for the amateur with a superb flavour, unfortunately it is a martyr to mildew, canker and scab. The tree was about twelve feet tall and about twenty feet across which made spraying difficult and picking off infected leaves impossible. I reasoned that Crown Gold was infecting the whole orchard. Last year, 2011, very few fruits were not infected. Crowngold had to go, and so it fell to the chainsaw last winter.

The next task is to remove the infected leaves, which can be tackled on several fronts. Blistered leaves should be picked off the tree as soon as possible to prevent the release of spores and destroyed. Rake or pick up all dead leaves, since these harbour over- wintering spores which will erupt in spring and then, spread by the wind, settle on foliage and shoots. Leaf litter can be sprayed with urea, which accelerates its decomposition, or the leaves chopped up, so that worms are able to pull the smaller leaf parts down into their burrows. When picking up leaves I frequently find huge leaves partially stuck in the entrance to a worm’s burrow.

All round good husbandry is important. Prune out infected wood scab, which appears as lesions and can erupt years after. Finally, plant on a well ventilated site and choose scab resistant varieties.

Scab can be controlled by spraying with the fungicide ‘systane’, as commercial fruit growers do in the spring. Amateurs may buy this in handy 300 ml containers. Last year, 2011, I chose a programme of spraying with garlic extract, which is claimed to have a controlling effect on scab. The garlic spray was applied from March every ten days until the beginning of June. But from then on until the second week of July, I sprayed every seven days as it was obvious by the end of May that it was not going to work in my orchard and a great disappointment.

Some scab susceptible varieties of apple and pear, I find, respond very well to clearing up the dead leaves under the trees, although some do not. Removing the dead leaves, merely under or around the canopy reduced or resulted in no scab on the apple Falstaff and pears Williams’ Bon Chrétien and Onward. All three are notable scab factories, which points to a cycle of infection and re-infection year on year. The source of scab infection can be very local and so local as to be under the tree, or it may be in the next garden. Over the years I have noticed that my scab problems appear to be worst when the wind is stuck in the north east and where there are several old Bramley trees belonging to a neighbour.

In my experience, an apple variety’s susceptible to scab and, therefore, one that cannot not be recommended for wet areas or those in which the movement of air is poor include: Spartan, Red Devil, James Grieve, Rev W Wilkes, Courtland, Fortune, Winter Gem, Delbarestivale, Pinova, Limelight, Saturn and Golden Delicious and Tentation.

Pears very prone to scab in my orchard are Louise Bonne of Jersey, Beurré Dumont, Delwilmor (Invincible), Onward and Williams’ Bon Chrétien

Pears which are scab resistant with me include: Joséphine de Malines, Concorde, Beth, Winter Nélis, Catillac (cooker), Black Worcester (cooker), Durondeau, Beurré Bedford, Doyenne d’Eté, Beurré Papa La Fosse, Beurré Precoce Morettini and, contrary to all the books, Pitmaston Duchess. I have others also that rarely have scab.

This year, 2012 was one of the worst years for scab. With endless low level cloud and rain, the trees were just about forever wet and if there was dry spell it was so brief that by the time I sprayed the last tree the heavens opened up and washed it all off. No amount of cleanliness prevented wholesale scab infection, but at least I can confirm that the apple varieties which remained free or had negligible scab damage were Charles Ross, Lord Lambourne, Ingrid Marie, Blenheim Orange, Finkenverder Prince (a German dessert variety, season October/November) and Dabinette, a classic cyder variety.

Pears were a disaster this year and frost wiped out the pears for most people across the country. My scab resistant croppers in 2012 were again Concorde and Durondeau planted against a wall, and these two never need spraying.

Adrian Baggaley

Photo courtesy John R. Hartman. Reproduced with permission from Vaillancourt, L. J. and J. R. Hartman. 2000. 'Apple scab.' The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-1005-01. Updated 2005.