How to Identify, Control, and Prevent Apple Scab

Apple scab (Venturia inaequalis) is one of the most economically damaging diseases for apple growers worldwide. In parts of Australia and Asia, this disease is also more commonly known as black spot disease. This fungal pathogen particularly thrives in areas with cool temperatures and wet spring weather. Although the geographic range of apple scab is widespread and in all parts of the world apples are grown, this disease is most severe in temperate climates. 

Apple scab is characterized by defoliation of the host trees, which can significantly weaken their overall vigor and fruit production. In addition, apple scab renders fruits unmarketable as it causes fruits to crack and opens the plant to secondary infections from other pathogens or pests. This pathogen results in more losses than any other plant disease that impacts apple cultivars.

In addition to apples grown for fruit, apple scab also impacts ornamental crabapple trees and mountain ash. For ornamental plants, their defoliation would negatively affect the overall look of these cultivated spaces. 

The primary control methods for this disease are using cultural and sanitation practices in conjunction with some fungicides for commercial growers. The timing for the application of fungicides is critical as fungicides are preventative for the disease and not a treatment. Finding the best organic fungicide that does not adversely impact the health of the trees or beneficial insects continues to be a challenge. 

What Is Apple Scab?

Apple scab is a disease caused by the fungus Venturia inaequalis. This disease causes defoliation and lesions on the leaves, stems, blossoms and fruits of affected plants.

V.inaequalis undergoes both sexual and asexual reproduction. The word “inaequalis” in its scientific name refers to the unique and unequal “footprint” shape of its sexual spores or ascospores. 

Life Cycle

The apple scab fungus overwinters on the fallen leaves and fruits of infected trees. During late spring and early winter, the fungus produces black erumpent pustules that kick starts the disease cycle by creating and emitting sexual spores (ascospores). The start of its sexual reproduction disease cycle coincides with host trees breaking domancy.

Wind and rain carry these spores to infect newly emerging leaves and nearby trees. Scab lesions start to appear within 10 days of infection and there is only one sexual reproduction cycle per year.  

Over the course of the summer, V.inaequalis sporulates repeatedly on leaves and further spreads the disease to other parts of the plant and to its neighbors. These secondary sporulations are the result of the asexual reproduction of conidia spores. Clusters of conidia spores give the lesions a velvety brown or olive green appearance.

Both types of spores require moisture to germinate and leaves or fruits must stay wet for a number of hours in order for an infection to occur. The length and severity of apple scab infection periods and the disease development depend on temperature, moisture, health of the host tree and and the ages of the leaves. Lesions spread less quickly on certain apple cultivars than others, which provides them with some levels of resistance to this pathogen.

Symptoms

This disease first appears in early spring on the bottom of young leaves as a fuzzy and undefined patch. Many people mistaken apple scab for mold or even just natural fuzz from the leaf. However, overtime, the fuzz becomes olive green spots and then more visibly brown lesions and eventually, turning them yellow and causing leaves to drop.

Defoliation of severely infected leaves is a key symptom of this disease which has detrimental effects on the health and growth of the infected tree, especially newly transplanted young trees. 

Lesions can also appear on fruits, although it is more unlikely for developing fruit to drop as with the leaves. Overtime, small lesions take on a defined border and a corky look. Lesions may lead to cracking on fruits. Primary infections from apple scab can open the fruit or leaves up to new infection from other diseases or pests. Sometimes, late seasons fruits will also not show visible symptoms of apple scab until they are in storage. 

Another apple fungal disease, cedar-apple rust, can also cause lesions on infected leaves. However, these lesions are yellow with a dark center. The dark center is where you can see the fruiting bodies of the fungus. Diseased leaves of host plants that are impacted by apple scab will look very different. 

Infections can also lead to lesions and openings on twigs. These openings are entry points for other diseases like the apple canker fungus. 

What Plants Does Apple Scab Affect?

Many apple and crabapple trees (Malus spp.) are susceptible to this scab. Specifically, popular varieties such as McIntosh, Cortland and Honeygold are very likely to be infected.  

Apple scab also affects mountain ash (Sorbus spp.), hawthorn (Crataegus spp.) and Cotoneaster (Cotoneaster spp.).  A similar fungus, Venturia pirina, infects pear (Pyrus communis). 

Control

As with many diseases, effective management of this scab requires a combination of techniques including cultivar selection, chemical controls, and cultural controls, as well as proper sanitation techniques. The good news is that apple scab rarely kills trees, except in the cases of very young trees that are experiencing multiple, major defoliations.

Because apple scab is a fungal disease, it thrives in damp conditions. Select a planting site that has at least six hours of sunlight per day so apple leaves have time to dry off. Do not use overhead watering so the leaves can’t get unnecessarily wet. In addition, pruning apple trees can improve the airflow around leaves and fruits.

Sanitation is very important when managing apple scab and to minimize the spread of the fungal spores. Remove fallen leaves if possible and try to speed up the rate of leaf decomposition by mulch mowing and adding a nitrogen source like a 5% urea solution to speed up the process. Apply urea in late fall and after the last leaf drop as to not stimulate more leaf growth on the tree during the winter.

Finding a suitable organic fungicide has been a big challenge for farmers and researchers worldwide. Only sulfur and lime sulfur have been approved for organic use. However, the negative impacts of lime sulfur on apple trees have been widely documented. Elemental sulfur has been documented to harm beneficial insects, like predatory mites, that help to control other pest populations. 

Prevention

Organic apple growers plant resistant varieties to limit the impact of the apple scab fungus and the use of chemical controls. There are more than 25 scab resistant cultivars including Honeycrisp, Freedom, and Redfree. There are also multiple resistant varieties of ornamental crabapple trees including Don Wyman, Prairifire and Snow Drift.

Planting resistant cultivars is a great method to prevent apple scab in your garden or orchard. While some cultivars are resistant to apple scab, they may still be susceptible to other apple diseases so there is no “cure-all”.