Apple scab 101

What is it and what can be done about it? By Anna Mouton.

Apple scab or black spot is a disease of apples caused by the fungus Venturia inaequalis — unaffectionately known to many growers as fusi — short for Fusicladium dendriticum.

Apple scab has been around for as long as there have been apples. Scab is thought to have co-evolved with apples in Central Asia and spread worldwide due to apple domestication and cultivation. Apple scab first appeared in the literature in 1819 but is depicted in art from as early as 1601 — see figure 1.

Signs of disease

The susceptibility of tissues to the apple-scab fungus decreases with age. New leaves and fruit are therefore at the greatest risk of infection.

Leaves initially show irregularly shaped light-coloured markings. These darken over time and appear velvety due to the formation of spore-releasing structures. Apples develop similar markings at first. Later these can become scabby and cracked. Severely affected fruit may be misshapen and drop prematurely.

Typical signs of apple scab on leaves and fruit are shown in the cover image.

Infected fruit may appear normal at harvest and then develop visible scab during storage. Scab lesions also increase the risk of post-harvest decay.

The consensus seems to be that apple scab is the most economically important fungal disease of apples. The economic impact of apple scab is largely due to the convergence of the intensification of commercial apple production with consumer demands for unblemished fruit.

 

Control

The two main strategies for apple-scab control are to reduce the number of spores landing on trees and to protect trees against those spores that do land. Growers tend to focus on the latter — but integrated disease management that targets all fungal vulnerabilities tends to be more cost-effective and sustainable than spray programmes in isolation.

Chemical control

Apple scab is most severe in areas where springs are cool and moist. Ascospore release is triggered by rainfall and the risk of successful infection depends on the combination of rainfall and temperature. Susceptibility to infection is greatest in new growth and declines as plant organs mature.

Prevention of primary infections in spring is critical as this breaks the life cycle. Primary infections allow the fungus to gain a foothold from where it can go forth and multiply rapidly all summer long. This results in large numbers of infected leaves dropping in autumn, ultimately boosting sexual reproduction during winter resulting in greater numbers of infective ascospores released the following spring.

Predictive models aim to identify the best times for fungicide applications based on weather data. Read more about these in the other articles in this package.

The apple-scab fungus has developed resistance to several classes of fungicides. Development of resistance can be slowed by applying fungicides less frequently, alternating applications of fungicides from different classes, and integrating spray programmes with other disease control measures such as orchard sanitation and leaf shredding.

Resistant apple cultivars

The promise of scab-resistant apple cultivars has stimulated several breeding programmes. Unfortunately, these have struggled with two main challenges, the first being that many scab-resistant cultivars cannot compete with non-resistant cultivars on fruit quality and storability.

The other problem has been the ability of the apple-scab fungus to evolve ways to overcome resistance in apple trees. Most scab-resistant cultivars rely on the Vf gene derived from the Japanese flowering crab apple, Malus floribunda. The first resistant plants were bred more than a century ago. Within less than 70 years, the first scab was observed on Vf cultivars abroad.

Fortunately, the Vf gene continues to protect against South African strains of apple scab. Trials are also ongoing to test the efficacy of other resistance genes against South African strains.

Orchard sanitation

The apple-scab fungus primarily overwinters on fallen leaves. It spends the winter sexually reproducing and creating countless ascospores that can infect trees in spring. The obvious advantage of attacking the fungus in winter is that it disrupts the life cycle of the pathogen, thereby reducing the number of infective spores released in spring. But there is another reason to target the overwintering fungus.

Sexual reproduction is how the apple-scab fungus — and everything else that reproduces sexually — creates genetic diversity. Genetic diversity leads to new strains of fungus that can overcome chemical control and resistant cultivars. So stopping apple-scab sex matters.

The simplest way to remove overwintering fungi is to reduce leaf litter by removing leaves, or by accelerating their decomposition and incorporation into the soil. Read more about relevant local research in the accompanying article.

Leaves can also be treated with fungicides or biologicals. Some researchers have reported that spraying with urea in autumn suppresses fungal reproduction while speeding up leaf breakdown.

Biological control

Biological control agents have the potential to reduce the need for fungicides. Researchers have identified fungi that can inhibit the growth and spore production of apple-scab fungus. These have not yet been commercialised, but may become extra weapons in the ongoing war on apple scab.

Bonus: Navigating the names of the apple-scab fungus

Many growers know the apple-scab fungus as Fusicladium dendriticum. But the correct name is Venturia inaequalis. So why has the apple-scab fungus had more than one name? Because of the complicated sex life of the group of fungi — the Ascomycota — to which it belongs.

Ascomycetes can reproduce both sexually and asexually. They have different reproductive structures depending on whether they are in a sexual or asexual stage. So the same fungus can have two forms and basically look like two different fungi which then end up with two different names.

Researchers prefer to classify fungi according to sexual reproductive structures. But they are frustrated by some fungi — known as fungi imperfecti — of which the sexual structures have never been seen. So how can researchers name them?

The solution used to be that the sexually reproducing stage of a fungus had a different name from the asexual stage. Venturia inaequalis was the sexually reproductive stage of the apple-scab fungus. Fusicladium dendriticum was the asexually reproductive stage.

Spilocaea pomi is an older name for Fusicladium dendriticum which is still sometimes used — just to add to the confusion.

In 2011, the international body that makes rules for naming algae, fungi, and plants decided that enough was enough, and adopted the principle of one fungus, one name. Although mycologists continue to argue about the names of many fungi, they have settled on Venturia inaequalis as the correct name for the apple-scab fungus — at least for now.