Apple Russet Control: How To Prevent Russeting Of Apples


By: Liz Baessler

Russeting is a phenomenon that affects apples and pears, causing slightly harder patches of brown on the skin of the fruit. It doesn’t harm the fruit, and in some instances it’s actually considered a feature, but it’s not always welcome. Keep reading to learn more about apple fruit russet and means of apple russet control.

What is Apple Russeting?

Apple fruit russet is the brown scarring that sometimes appears on the skin of the fruit. It is a symptom rather than a disease, which means it can have several different causes. One of the most common causes of apple russet is genetic tendency. Some varieties are so prone to it that they actually get their name from it, like Egremont Russet, Merton Russet, and Roxbury Russet.

Other varieties like Pippin, Jonathan, and Gravenstein, while not named for it, are still very prone to apple fruit russet. If you’re uncomfortable with russeting, avoid these varieties.

Other Causes of Apple Russet

Although it’s naturally occurring in some apple varieties, the russeting of apples can also be a sign of more serious problems like frost damage, fungal infection, bacterial growth, and phototoxicity. Its presence is a good sign to check for these problems.

Yet another cause of apple russeting is a simple case of high humidity and poor air circulation. (And it’s conditions like these that often lead to the more serious problems listed above).

Apple Russet Control

The most effective method of prevention is to keep trees well spaced and reasonably pruned, with a strong but open canopy that allows good airflow and sunlight penetration.

It’s also a good idea to thin the fruits themselves to 1 or 2 per cluster soon after they start to form in order to keep moisture from building up between them. Try to opt for varieties that are not known for russeting, like Honeycrisp, Sweet Sixteen, and Empire.

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Roxbury Russet

The 'Roxbury Russet' is an apple cultivar, believed to be the oldest apple cultivar bred in the United States, having first been discovered and named in the mid-17th century in the former Town of Roxbury, part of the Massachusetts Bay Colony southwest of (now part of) Boston. [1] It is known by several other names including 'Boston Russet', 'Putnam Russet', and 'Sylvan Russet'. [2]

It is a greyish-green russet apple known for its good winter-keeping qualities, as well as its suitability for making cider and juice. It is not widely grown or commercially available due to general commercial disfavor for russet varieties the dull and heavily marked face makes it hard to sell now. [1] The yellow-green flesh is firm and coarse-textured, suited for eating fresh and cooking. [1] It is available from growers who specialize in heirloom plants. It ripens from September to October, [3] and so is commonly available in autumn in farmers markets in the Northeast. Each apple contains 12.87% sugar that ferments to 6% alcohol in hard cider production. [4]

Propagation wood of 'Roxbury Russet' (it propagates by grafting) was taken to Connecticut soon after 1649. [2] Thomas Jefferson planted a number of 'Roxbury Russet' trees in Monticello's South Orchard in 1778. [5]

The Roxbury Russet apple was one of the varieties grown by Major General Israel Putnam on his farm in Pomfret, Connecticut. His grandson, also named Israel Putnam, introduced this variety to the Ohio Valley in 1796. The grandson received a total of 23 varieties of apple from Connecticut in that year, most of which probably came from his grandfather's farm. The Putnam Russet (Roxbury Russet) was considered to be the best and most profitable winter apple of all the varieties received, and was regarded as a good "keeper" (an important characteristic in an age before refrigeration). [6] And not only a good keeper, but if we are to believe Nathaniel Hawthorne, one that improves with age. For in The House of the Seven Gables, Uncle Venner remarks, “But I suppose I am like a Roxbury russet, – a great deal the better, the longer I can be kept.”


Michele Warmund
University of Missouri
(573) 882-9632
[email protected]

Published: May 21, 2019

Russet is a rough, tan skin defect that commonly occurs on some apple cultivars in Missouri, such as Golden Delicious, Golden Russet, Ginger Gold, Rome, Idared, Empire, Stayman, Cortland, Jonagold, and McIntosh (Figure 1). However, late-maturing strains of Golden Delicious sometimes have less russet than early-maturing strains. In contrast, Red Delicious is relatively resistant to russet. While russeted fruit is unattractive, it is safe to eat. Russeted apples can be salvaged by simply peeling the fruit or using them for juice or cider.

Figure 1 Russeting on clusters of Golden Delicious apples that were not properly thinned. Note that the russet is most severe where moisture or chemical spray collected after application for a prolonged period of time.

Russet symptoms develop when there are cracks in the cuticle of the fruit. Epidermal cells underneath the cuticle become injured, turn brown, and are then pushed upward to become exposed at the peel surface as the fruit develops. Russet injury occurs from the pink stage of floral development to the first 40 days after petal fall on apple. Often the side of the fruit that faces the interior of the tree canopy has more severe russeting, which is often associated with the accumulation of moisture on apples and slow drying conditions.

Unfavorable weather conditions, such as cool temperatures, prolonged cloud cover, rainfall, and heavy dew, promote russet formation. This type of weather may also contribute to fungi that can russet fruit following infection. Aureobasidium pullulans and Rhodotorula glutinis are two types of fungi commonly found on apple fruit surfaces and foliage that cause russeting. Also, the fungus that causes powdery mildew, Podosphaera leucotricha, produces russet on susceptible cultivars, such as Jonathan and McIntosh.

Fungicides can also promote the development of russet when they penetrate the fruit cuticle. Copper, captan, sulfur, and liquid lime-sulfur products can cause phytotoxicity, especially when trees are sprayed in the evening, when high humidity occurs and there is low wind speed. When using copper for fire blight suppression, apply it no later than the green tip bud stage (when leaf tips are just visible). Also, copper applied later in the summer can result in blackened lenticels on fruit. If summer copper sprays are applied, use low-rate copper products such as Phyton or Cueva under rapid-drying conditions when the foliage is dry. Another way to minimize russeting on susceptible cultivars is to omit Captan from sprays and use a different fungicide from bloom through 40 days after petal fall. Consult The Midwest Fruit Pest Management Guide at https://ag.purdue.edu/hla/hort/documents/id-465.pdf for alternative fungicides.

For organic growers, avoid using sulfur or liquid lime sulfur products when the temperature at the time of application or for the next four days will be over 85°F. During hot weather, substitute low-rate copper products (Cueva, Previsto) for those containing sulfur. Although the low-rate copper products may induce some russet when applied during warm temperatures, they will cause less injury than sulfur products sprayed under similar conditions.

Commercial products containing GA4+7 (gibberellic acid), such as ProVide and Novagib, can mitigate russet injury. For this use, the product is applied four times at 10-day intervals. However, the high cost of these products may not justify their use, depending on the apple cultivar and its use.


Cedar-apple rust

There are several cedar-rust diseases that spend part of their life cycle on Eastern red cedar (Juniperus virginiana) and other junipers, and another part of their life cycle on apple, hawthorn, and other members of the rose family. Both hosts are required for the fungus to complete its life cycle. The three most common rusts occurring in Illinois are caused by Gymnosporangium juniperi-virginianae (cedar-apple rust), G. globosum (cedar-hawthorn rust), and G. clavipes (cedar-quince rust).

DISEASE CYCLE

The rust organism spends one full year of its life cycle on junipers. During the second spring, usually around the time crabapples are in bloom, the galls become rain soaked and swell, producing jelly-like tendrils (spore horns) that project out of the galls. As the spore horns begin to dry, the spores are released and carried by the wind to young, newly developing leaves of hawthorns and other susceptible plants. Dispersal of spores can range up to 5 miles from a juniper but most infections develop within several hundred feet. About a month after crabapples have bloomed, the spores are exhausted and most leaves are no longer susceptible. Ten-to-14-days from initial infection, small yellow spots can be seen on upper surfaces of infected leaves. Several weeks later, the fungus appears as orange or brown spots with hairlike appendages on the underside of the leaf. In late summer, the rust spots release the spores and are carried to nearby junipers.

Cedar-apple rust is the most common of the three fungal rust diseases and attacks susceptible cultivars of apples and crabapples. It infects the leaves, fruit, and, occasionally, young twigs. The alternate host plant, Eastern red cedar (Juniperus virginiana), is necessary for the survival of the fungus. Spores produced on rose family plants only infect juniper plants, and those originating on the evergreen host only infect rose family plants. Repeated infections of cedar-apple rust can be unsightly and seriously weaken and destroy the ornamental value and health of susceptible plants.

Symptoms on Apple and Crabapple

On leaves: Bright yellow/orange spots develop on the upper surface of the leaves in late spring. These spots gradually enlarge, becoming evident on the undersurface of the leaves as small bulges. In midsummer, these rust lesions develop hairlike, cylindrical tubes (hyphae), which release spores into the air that are blown to the juniper host. Infected leaves of apples and crabapples may drop, with defoliation more severe in dry summers. Galls that form on the juniper host do not become evident until July the next year, requiring two years for the fungus to complete its life cycle.

On twigs: The rust appears as a swollen corky gall on the current year’s growth, usually no more than 1 inch in length. The swelling eventually develops the characteristic cylindrical fruiting bodies. Seriously affected twigs are stunted and may die.

On fruit: The rust causes yellow to orange spots similar to those found on the leaves, but the spots are usually much larger. Fruit infection causes an inferior fruit quality or premature fruit drop.

Symptoms on Juniper

In mid-spring, swellings or galls develop on juniper needles that were infected with spores during the previous year. These galls are brown to dull red in color, globular in shape, and may vary from pea-sized to an inch or more in diameter. As they mature, circular pits or depressions are found over the surface of the galls. After spring rains and damp weather, yellow gelatinous tendrils or spore horns form in these pitted areas. The tendrils elongate rapidly and release spores during dry, windy weather that follows the spring rains. Spores produced on the juniper host are then blown to the apple, crabapple, and hawthorn hosts as their new growth emerges.
Eventually the galls dry out but remain attached to the tree for several years, resulting in some small twig and tip dieback.

CEDAR-HAWTHORN RUST

Cedar-hawthorn rust is very similar to cedarapple rust, both in appearance and occurrence, but infects a broader range of plants within the rose family. The severity of the disease is usually minor on crabapples and apples (Malus sp.), mountain ash (Sorbus), and pears (Pyrus), but can be quite serious on many hawthorns (Crataegus sp.).

Symptoms on Hawthorn

On leaves: Large yellow spots appear on cedar-hawthorn rust the upper surface of the leaves turning yellow orange to gray-brown as the spores mature. When rust is severe, all the foliage may turn bright yellow and drop prematurely. The orange leaf spots are smaller on apple and crabapple.

On fruits and twigs: Deformation of fruits and young twigs is particularly severe on hawthorns, but this damage is usually caused by the cedar-quince rust fungi and not cedar hawthorn rust fungi.

On juniper: Cedar-hawthorn rust galls are smaller in size than cedar-apple rust galls, less symmetrical, and more chocolate-brown in color. Galls remain on the twigs of branches of junipers for several years, where they continue to produce spores, compared to the one season spore production of cedar-apple rust.

CEDAR-QUINCE RUST

Cedar-quince rust affects quince (Chenomeles), serviceberry (Amelanchier), hawthorn (Crataegus), mountain ash (Sorbus) and many other plants in the rose family. Though generally not as prevalent as cedar-apple rust, it causes the greatest amount of damage to the fruits, twigs, and thorns of susceptible plants. During extended periods of wet weather, when temperatures range between 50 degrees F and 75 degrees F, severe infection can occur just four hours after initial leaf contact.

Symptoms of leaves: Basically none, occasionally veins or petioles will be swollen.

On twigs and thorns: Elongated swollen cankers appear on twigs and thorns. In damp weather, orange to brown spores are visible.

On Junipers: Spindle-shaped swelling occurs on twigs and branches of junipers. Young branches are usually girdled, then die. In damp weather, older galls are covered with masses of gelatinous, orange to brown spore horns. Galls can produce spore horns for 4 to 6 years, sometimes longer.

DISEASE MANAGEMENT

Cultural Control
Because this disease requires two hosts, the separation of the hosts for a distance of one mile will help reduce infection. Ideally, to minimize disease host availability, plant trees and shrubs that are resistant to rust diseases. There are many apples, crabapples, hawthorns, and junipers that exhibit resistance to these diseases. The Morton Arboretum publication Crabapples for the Home Landscape provides information on selecting crabapples.

Chemical Control
Protective fungicides can be applied to help minimize infection. A minimum of three applications should be done. These applications protect the new leaves from spores that are dispersed from the juniper host in mid-spring. Spraying apple, crabapple, and hawthorn foliage after symptoms develop has no controlling effect.
Apples and Crabapples: Begin spraying when new growth appears and flower buds show color but are not yet open. Repeat three to four times at 10 to 14 day intervals.

Hawthorns:
Spray as new growth appears and flower buds begin to open. Repeat 3 to 4 times at labeled intervals. Washington hawthorns are very susceptible to quince rust and form noticeable stem cankers that should be pruned out.

Junipers:
Begin spraying susceptible plants in early July and continue at labeled intervals through August. Remove galls and cankers to reduce infection of alternate hosts.


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