Cedar-Apple Rust Facts

Over the last two weeks I’ve observed two young Pixie Crunch apple trees in our orchard succumbing to cedar-apple rust. Or so I suspect. (Rosslyn Redux)

So what do you think? Cedar-apple rust? Something else? Although I dread admitting it, I’m fairly convinced that we’re battling a light invasion of cedar-apple rust which has undoubtedly evolved quite happily, unimpeded in the old meadows, volleying back and forth between the native cedars and old abandoned apple trees.

To brace myself, I’m digging into the nitty-gritty details, learning what I can about organic cedar-apple rust treatments, and culling the Eastern Red Cedars (Juniperus virginiana) growing nearby, the very ones upon which I’ve detected the telltale galls and gelatinous orange horns ever since we started landscaping, gardening and recovering the back meadows.

When I have time to thumb through old photos from the spring/summer of 2008 or 2009 I’ll dig out some of the photographs I took. At the time I was fascinated with the colorful fungi that emerged after wet periods. I snapped images to help me identify what I took for an innocuous parasite.

Symptoms of Cedar-Apple Rust

In my opinion, the symptoms of cedar-apple rust are most evident on the cedar trees, especially during the brightly colored phase visible in the photo on the right.

On the Eastern Red Cedar host, the fungus produces reddish-brown galls from 1/4 to 1 inch in diameter. These galls can be mistaken for cone structures by the uninitiated. After reaching a diameter of about 1/2 inch, they show many small circular depressions. In the center of each depression is a small, pimple-like structure. In the spring these structures elongate into orange gelatinous protrusions or horns. The spore-bearing horns swell during rainy periods in April and May. The wind carries the microscopic spores to infect apple leaves, fruit and young twigs on trees within a radius of several miles of the infected tree. (Wikipedia)

Perhaps because my apple trees afflicted with cedar-apple rust are still small, the fungus is less apparent. But closer examination of the leaves (no fruit have set on the pair of young Pixie Crunch apple trees yet) reveal the signature markings.

The most conspicuous symptoms on apple are bright orange, glistening lesions on the leaves. Lesions which are not inhibited chemically may form small tufts of spore-producing structures (aecia) on the lower surface of the leaf by July or August. Cedar-apple rust appears on fruit first as bright orange, slightly raised lesions, but may take on a more brown and cracked appearance as the fruit enlarges. Usually some of the orange color remains at harvest as evidence of the early season infection… Stem infection causes a slight swelling of the stem and may result in abscission of the young fruit. (West Virginia University, Kearneysville)

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. (The Morton Arboretum)

Disease Cycle of Cedar-Apple Rust

I’d like to dive a little deeper into the complex relationship between cedar-apple rust (Gymnosporangium juniperi-virginianae) and the two host species – apple trees and eastern red cedar trees – that sustain it.

Operating well out of my arena of even nominal knowledge, I’ll continue to defer to the experts as we examine the 2-season life cycle of cedar-apple rust.

I apologize for redundancies, but as a non-expert I feel better include too much rather than too little information, even at the risk of overemphasizing some aspects of the condition. If you’re a quick study, scan and move on.

The rust organism spends one full year of its life cycle on junipers. During the second spring… 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. (The Morton Arboretum)

Cedar Apple Rust is most easily identified by the appearance of small yellow to orange lesions that will appear on the top of leaves, petioles, and even on young fruits. Depending on host susceptibility, these lesions can increase in size at varying rates, with faster enlargement on more susceptible cultivars. These lesions can occasionally be surrounded by a red band, but this is not the standard. After this, small brown pustules will develop that are no larger than 1mm in diameter. These will produce watery, orange drops. Next… comes yellow brown lesions up to 15mm in diameter on the underside of leaves. From these, dark tubular structures are produced; these will release red brown spores. (University of Illinois, Urbana-Champaign)

Cedar apple rust is caused by the fungi… Gymnosporangium juniperi-virginianae that spend part of their life cycles on Eastern Red Cedars growing near orchards. The complex disease cycle of cedar apple rust, alternating between two host plants… [starts when] at the first warm rain of spring, the spore horns become gelatinous masses and produce their teliospores.

Wind carries the spores to apple leaves… [where they] attach themselves to the young leaves, germinate, and enter the leaf or fruit tissues.

Wind carries the spores to apple leaves at about the time that apple buds are in the pink or early blossom stage. Upon reaching apple buds or leaves, the spores attach themselves to the young leaves, germinate, and enter the leaf or fruit tissues. Infection takes place in as little as four hours under favorable conditions. Yellow lesions develop in one to three weeks.

In July and August, spores from the apple leaves (Aeciospores) are produced. The wind carries the spores back to Eastern Red Cedars, completing the infectious cycle. The spores land on cedar needle bases or in cracks or crevices of twigs. There, they germinate and producing small, green-brown swellings about the size of a pea. Galls do not produce spores until the second spring. However, mature galls usually are present every year… (Wikipedia)

Plenty of overlap between these three sources (or should I say “millions of sources” since Wikipedia is a gargantuan, open source collaboration?) A pretty clear picture is emerging, and a none too enticing picture at that!

Prevention & Treatment of Cedar-Apple Rust

So for a gardener who avoids non-organic pest control, cedar-apple rust appears to be a rather formidable adversary. And yet, I’m hoping to bypass the threat without resorting to harmful chemicals. As I mentioned above, I’ve already begun to eliminate cedars within the immediate vicinity, and we’ll continue culling all withing a quarter mile or so of the orchard. This is an especially reasonable preventative measure because we can sue all of the cedar trees as naturally rot-resistant fence posts, and we can chip the branches into mulch. This is known as “cultural control”.

The easiest (or maybe just most successful) practice would be to remove all the galls from Eastern red cedar trees in the surrounding areas. This rust does not overwinter on any of the hosts mentioned in this article, and instead overwinters on nearby cedar. Midway through the growing season, the yellow orange spots release spores that will infect cedar to ensure the pathogens winter survival. The cedar galls are bright orange in color, and look like strange masses of jelly tendrils. These should be removed in early spring to prevent summer infection, and then removed once again in the three to four week blooming period.. (University of Illinois, Urbana-Champaign)

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. (The Morton Arboretum)

Interruption of the disease cycle is the only effective method for control of the cedar apple rust. The recommended method of control is to “remove cedars located within a 1-mile radius” of the apples to interrupt the disease cycle, though this method is seldom practical… [Planting less susceptible apple tree varieties is prudent, because] resistant varieties are less susceptible to attack, but that does not mean that they are free from an aggressive attack. (Wikipedia)

While a mile radius is challenging, I am removing all red cedars with open lawn or meadows allowing easy spread of the cedar-apple rust spores. And I will ensure that all new apple trees planted this fall and next spring are not overly susceptible to the affliction.

Plant resistant varieties. (Clemson University)

Sometimes the simplest solution is best! I will follow this advice going forward, even though I am also hoping to eradicate the cedar host. And with a stroke of luck I’ll be able to avoid the most frequent treatment solution proposed: “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… foliage after symptoms develop has no controlling effect… Begin spraying when new growth appears and flower buds show color but are not yet open (balloon stage). Repeat three to four times at 10 to 14 day intervals. (The Morton Arboretum)

Fungicide sprays applied in a timely manner are highly effective against the rust diseases during the apple cycle… If cedar apple rust disease is diagnosed on apple fruits and leaves it is far too late to spray.

Application of fungicides to the junipers before and while they are in the infectious orange gelatinous state seems to reduce the severity of the outbreak. (Wikipedia)

To date I’ve been unable to identify any truly organic fungicides guaranteed to treat cedar-apple rust. They may exist, and despite my optimism that I’ll overcome the threat posed by this fungus, I would be interested in hearing from you if you’re aware of a proven organic treatment. Thank in advance for your assistance.