Cedar apple rust is a heteroecious, macrocyclic rust disease caused by the fungus Gymnosporangium juniperi-virginianae. The pathogen usually infects members of the genus Malus, including apple, Malus domestica, and crabapple Malus spp.
Eastern red cedar Juniperus virginiana and other juniper species are also susceptible to infection. The disease is highly distributed in North America and occurs wherever both the susceptible juniper and the susceptible apple species arise.
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Causal organism of Cedar Apple Rust
Gymnosporangium juniperi-virginianae is the causal organism and belongs to family Pucciniaceae, order Pucciniales, Class Pucciniomycetes and phylum Basidiomycota.
Gymnosporangium species are obligate biotrophic pathogens, meaning they depend on the existence of the living host tissue to complete their life cycle. Since the Gymnosporangium juniperi-virginianae species belongs to the Pucciniaceae family, it would produce spores at some stages of its life cycle with specific morphology and infection functions depending on the host’s juniper and apple.
Symptoms of Cedar Apple Rust
On apples, the disease appears first on leaves as small yellow-greenish spots that enlarge to become orange-yellow and gradually change and become surrounded at borders by concentric red bands. On the upper leaf surface, black specialized fruiting structures called spermogonia are seen. On the underside, the lesions are formed which are known as aecia. The leaf thickens around this structure making it a cup-like appearance.
On fruits, yellow lesions are seen which are usually larger than that of leaves and the fruits are usually stunted and misshapen and may drop quickly.
On cedar, the needles are infected during the summer month and following summer, small greenish-brown swellings appear on the upper or inner foliage structure. These swellings enlarge during autumn and appear as kidney-shaped galls and each gall is covered with small cellular depressions. In these galls, gelatinous materials called telial horns are seen.
Disease Cycle of Cedar Apple Rust
The disease cycle of G. juniperi-virginianae is pretty complex, i.e. it is almost two years with five macrocyclic spore stages and two obligate hosts, placing the fungus in the heteroecious category. Each stage plays a specific role in the pathogen reproduction and spread as well as has specific interaction between fungus and its host plants.
The fungus overwinters as mycelium on the galls of juniper and the stages begin from springtime.
1. Telial Stage
Telial stage begins in springtime and occurs within the host Juniperus sp. After their dormancy phase over winter, the brownish, woody galls developing on juniper trees swell to produce gelatinous horns, which are known as telial horns. The horns develop in response to moisture, which comes very frequently as bright orange, gelatinous structures.
The telial horns are comprised of teliospores, which are dikaryotic spores. They germinate to form haploid basidiospores through meiosis. The basidiospores are wind-blown over distances of up to several kilometers, depending on environmental conditions.
2. Spermatial Stage
Once the basidiospores have landed on the leaves of apple or crab apple trees, they infect by penetration through stomata or direct penetration, initiating small, chlorotic lesions.
Initially, the infections are symptomless, but several weeks later, they can be seen as lesions. This in turn leads to the development of spermogonia on the upper side of the leaves, which are known as pycnia.
Spermagonia produces haploid spermatia (male gametes) and receptive hyphae (female games). Fertilization comes about through the transfer of spermatia from one infection site to the receptive hyphae of another by insect vectors or by rain splash. The dikaryotic mycelium is now formed which will produce more spores.
3. Aecial stage
It occurs on Malus sp. where the fungus enters the aecial stage. Aecia is formed in chains on the underside of leaves producing cylindrical aeciospores. These spores cause the bright yellow-orange lesions characteristic of cedar apple rust.
Lesions also can appear on fruit and twigs if the disease is severe, but infections occur most often as leaf infections.
Aeciospores are released in late spring or early summer and are wind-blown to the nearest juniper hosts, where they germinate and infect juniper tissue, completing the fungal life cycle. Unlike basidiospores, aeciospores can be transferred long distances sometimes miles, so the pathogen may move between widely separated hosts.
4. Teliospore Formation
The latent infections resulting from infection by aeciospores develop slowly over several months into galls. These galls, reaching about can be rather conspicuous by late summer or fall. The galls are quiescent during the winter but constitute the reservoir necessary for the survival of the pathogen overwinter. In the spring of the following year, they produce new telial horns to start the cycle over.
The specific environmental conditions favorable for the disease are:
- Temperature: The optimum temperature required for the infection and spore dispersion ranges from 10°C to 25°C. Cool and moist conditions lead to higher levels of infection.
- Water: Water is important in the release of basidiospores and, the emergence of telial horns; it also serves to splash spermatia onto apple trees.
- Host Proximity: Proximity between the juniper and apple hosts has a strong impact on the epidemiology of the disease. Short host distances enhance the possibility of severe infections.
Control and Management Strategies of Cedar Apple Rust
Cultural Control: The most efficient control measure is the removal of juniper trees within a 2-5 kilometer radius around apple orchards. This reduces the local source of basidiospores. The proper pruning of juniper trees regularly to remove the gall before the formation of telial horns reduces the risk factor of the pathogen.
Fungicides Application: Fungicides are effective in protecting apples from pathogens. Pre-infection treatments are given by protectant fungicides, such as mancozeb, captan, and copper-based compounds. For most of the post-infection treatments, systemic fungicides such as myclobutanil, trifloxystrobin, or tebuconazole are used.
Applications of fungicides should be timed to coincide with the release of basidiospores from juniper galls through petal fall from apple tree bud break.
Host Resistance: Several of the newest apple and crab apple cultivars, such as ‘Liberty,’ ‘Enterprise,’ and ‘Freedom,’ display strong resistance to cedar apple rust and obviate much chemical control.
References
- Cedar Apple Rust. (n.d.). https://www.fs.usda.gov/wildflowers/plant-of-the-week/gymnosporangium_juniperi-virginianae.shtml
- Cedar-apple rust and related rust diseases. (n.d.). UMN Extension. https://extension.umn.edu/plant-diseases/cedar-apple-rust
- Stearns, S. (2021, August 23). Apple Rust Disease. Integrated Pest Management. https://ipm.cahnr.uconn.edu/apple-rust-disease/
- Sgs. (2024, June 18). Apple IPM – Cedar Apple Rust. Center for Agriculture, Food, and the Environment. https://ag.umass.edu/fruit/fact-sheets/apple-ipm-cedar-apple-rust
- Barnes, E. H. (1979). Cedar Apple Rust. In Springer eBooks (pp. 269–277). https://doi.org/10.1007/978-1-4684-3495-8_48
- Manrique, J. (n.d.). Apple rust diseases. Scribd. https://www.scribd.com/document/459195735/Apple-Rust-Diseases
- Vedantu. (n.d.). Rust. VEDANTU. https://www.vedantu.com/biology/rust
- Libretexts. (2021, October 13). 2.45: Rust fungi (order Pucciniales, formerly Uredinales). Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Botany/Inanimate_Life_(Briggs)/02%3A_Organisms/2.45%3A_Rust_fungi_(order_Pucciniales_formerly_Uredinales)
- Cedar-apple rust. (n.d.). https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/rusts/cedar-apple-rust
