Apple Scab, Venturia inaequalis

Illustration by Adelaide Tyrol

Although apple scab is everywhere all the time, it is most apparent in the fall, as we try to avoid apples spotted with its telltale, ugly, black lesions. Those spots, it turns out, were predetermined in the spring.

In order for the apple scab fungus to get a start, the leaf surface needs to be wet. Since spring is a popular time for rain, chances are good that all three elements of the so-called disease triangle are present: a susceptible host, the right climatic conditions, and the disease inoculum.

Venturia inaequalis, the fungus that causes apple scab, is an apple specialist. It only has one job, but it has fine-tuned its approach and is an expert at invading apples, and only apples. Except for varieties specially bred to resist apple scab, all apples are susceptible. Over the course of many decades, the breeders of scab-resistant varieties have been quite successful, and many familiar apples never (or almost never) have a black spot: Prima, Priscilla, Jonafree, Redfree, Liberty, Freedom, Goldrush, and Pristine are highlighted for their scab resistance in nurseries and nursery catalogs. And yet, although battles have been won, victory is not assured. There is always the chance that Venturia inaequalis, which reproduces sexually as well as asexually, will outmaneuver us humans.

Scab-resistant varieties are a good choice for the backyard, but commercial orchardists are looking for many different traits in an apple tree, not all of which are available in the varieties bred specifically for scab-resistance. Such favorites as Cortland, McIntosh, Empire, and Macoun, for instance, are highly susceptible to apple scab.

As a consequence, commercial growers work to manipulate the other two pillars of the disease triangle. No, they can’t change the weather, but apple growers can keep track of degree-days and rainfall, and by using charts developed over many years that predict when apple scab spores will be released, they can time the spraying of fungicides or sulfur to optimize their effectiveness. Just when new apple leaves unfold, the fungus, having spent the winter on the previous year’s fallen apple leaves, begins to produce spores, which then are blown to new leaves by the wind.

These spring spores, called ascospores, are released over a period of five to nine weeks, with the peak discharge at apple blossom time. Spores that germinate in May do so on the new leaves, and those infections produce asexual spores – called conidia – that cause new infections and produce, in turn, yet more conidia. Each initial lesion can produce up to 100,000 conidia over several weeks. Thus, apple scab is known as a polycyclic disease, with one early successful spore in spring capable of spawning an exponential series of infections. Badly infected leaves become shriveled and dwarfed and are not very useful to the tree. By the time untended apples ripen, they are often a mess.

Serious apple growers use a variety of strategies, all of which focus on eliminating early spring spores. In the fall, flail mowing will tear up fallen leaves, whereupon other decay organisms then move in and devour the leaf shreds, leaving nothing for an apple scab fungus to survive on over the winter. Autumn applications of compost, lime, or urea help the decomposition as well.

Chemically formulated fungicides are another approach, used either to destroy the fungus in the fallen leaves or to prevent it from invading new leaves. Organic growers depend on soil fungi, earthworms, and countless other soil organisms to keep apple scab under control.

One clever idea is to thoroughly soak the ground beneath apple trees when the forecast is for a couple of days of dry weather. Four hours of late morning watering will cause the spores to discharge, but they are doomed to failure if the apple leaves are dry.

Most of us delight in the fact that wild apples are abundant across the landscape, since they are such good food sources for wildlife, but for those who grow apples commercially, wild apples are the source of the countless spring ascospores that will be blown to the tended orchard where, over the course of the summer, they will duplicate themselves mercilessly, ruining fruits and decimating profits.

Unless the lesion has caused a crack in a developing apple, the ugly black spots are only skin deep and perfectly harmless. If, as shoppers, we developed a bit more tolerance for imperfection, it could make an apple grower’s life much easier.


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