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Beginner

Scientific Name: Arthuriomyces peckianus and Gymnoconia nitens

Identification

• Specific to brambles and occurs on blackberries, black raspberries, and purple raspberries
• In spring, infected plants produce a large number of spindly, clustered primocanes and new lateral shoots on fruiting canes.
• Leaves appear deformed and yellow.
• Fruiting canes do not produce blossoms and often lack spines.
• Infected plants develop waxy blisters on the underside of their leaves that turn bright orange and become powdery.
• By late May or early June, affected leaves begin to wither and die.
• New cane and leaf tissues appear healthy; however the disease is now systemic. 

Often Confused With
Late Leaf Rust

Period of Activity
Infected primocanes show early symptoms in the spring when leaves emerge. If not removed, leaves develop bright orange aecia on the underside of the leaves within 3 weeks. Infection is favoured by cool temperatures and prolonged leaf wetness. The disease overwinters on the underside of leaves, or within systemically-infected plants.

Scouting Notes
After leaves have just emerged in early spring, check new primocane growth for deformed, yellow leaves. Look for spindly, clustered new shoots that develop from the crown. Infected canes will also have few if any spines. Search the underside of leaves for waxy, blister-like pustules. Towards the end of May, search the underside of leaves for powdery, bright orange blisters. During the summer, check the underside of mature leaves for dark brown spots, especially in the lower canopy and toward the middle of the trellis.

Thresholds
None established.

 

Advanced

Scientific Name: Arthuriomyces peckianus and Gymnoconia nitens

Identification
This disease is specific to brambles and occurs on blackberries, black raspberries, and purple raspberries; however, it does not affect red raspberries

Orange rust of blackberry is a fungal disease that can be caused by either Arthuriomyces peckianus or Gymnoconia nitens. Although once considered to be part of the same species, significant differences in life cycles have been established and the two pathogens are considered distinct species.

A. peckianus occurs primarily on black raspberries, while G. nitens occurs primarily on blackberries.  Although most rust pathogens require two host species to complete their life cycle, these pathogens infect only one host species during their entire lifecycle.

In spring, infected plants produce a large number of spindly, clustered primocanes and new lateral shoots on fruiting canes. The leaves are deformed and yellow. Floricanes do not produce blossoms and the affected plants often lack spines. After a few weeks, the affected plants will show waxy blisters on the underside of their leaves. The blisters then turn bright orange and become powdery. By late May or early June, leaves affected by A. peckianus begin to wither and die.

Both fungal pathogens are systemic and perennial. Once a plant has been infected, the fungus will continually overwinter inside the crown and root tissue and the plant will not recover from the disease.  

New shoots and leaf tissue produced on infected canes after June will appear healthy and unaffected by the fungus. However, the disease is now systemic and the symptoms will recur again in the spring of the following year.

Often Confused With

Late Leaf Rust
Both late leaf rust and orange rust are fungal diseases, however, late leaf rust only affects red raspberries, while orange rust only affects blackberries, black raspberries, and purple raspberries.  Late leaf rust produces spores which are orange-yellow and powdery. Orange rust causes waxy blisters filled with bright orange spores on the underside of infected leaves.

Biology
Arthuriomyces peckianus produces all of the five spore stages, except for urediospores. Gymnoconia nitens has an abbreviated life cycle and produces neither urediospores nor aeciospores. Apart from the difference in the number of spore stages, both the lifecycles and disease symptoms caused by these two fungal pathogens are very similar.
In the spring, new leaf growth emerges from infected crown tissue. The overwintering mycelium forms spermagonia on the upper side of the unfolding leaves. At this point, the new growth appears spindly and the leaves are deformed and yellow. This makes it easy to identify infected plants and remove them before the fungus can spread further.  

After a few weeks, the underside of affected leaves begins to develop blisters, called aecia. These blisters are bright orange and appear waxy at first, but become powdery as they release aeciospores. In the case of black raspberries, the rusted leaves wilt and die after the release of aeciospores. The aeciospores can be wind-dispersed, but may also get spread by splashing rain. Between late May and early June, the aeciospores infect mainly mature leaf tissue in a localized fashion.

In order to germinate, the aeciospores require favourable environmental conditions. Temperatures between 6 and 22°C are optimal for germination, while temperatures above 25°C result in slow germination. When temperatures are above 30°C, germination and infection will not occur at all. More than low temperatures, the fungus requires free water in the form of high humidity and prolonged leaf wetness. After more than 6 hours of 100% relative humidity, the infection rate rises dramatically. Plants with a think canopy will retain moisture and create favourable conditions for aeciospores to germinate and infect mature leaves.

After 3 to 4 weeks, the underside of the infected leaf shows dark brown spots which are made up of telia. The telia contain the next spore stage, called teliospores. Teliospores are thick-walled and can remain dormant to survive harsh environmental conditions or to overwinter. They also give rise to basidia, which in turn bear basidiospores.

Basidiospores systemically infect buds of new shoots that grow from rooting cane tips. The fungal mycelium can now colonize the crown and root tissue in order to overwinter. In the case of G. nitens, the teliospores may not germinate to produce basidia in the fall, and instead serve as the overwintering stage. The overwintering teliospores remain attached to the underside of the leaves.  In the spring, they germinate to produce basidiospores which then infect buds of new shoots that grow from the crown.

Period of Activity
Infected primocanes show early symptoms in the spring when leaves emerge. If the affected plants are not removed at this stage, they will develop bright orange aecia on the underside of their leaves approximately 3 weeks after leaves emerged. By the end of May or early June, aeciospore dispersal occurs and mature leaves can become infected for a period of 2 to 3 weeks. Cool temperatures and prolonged leaf wetness, both of which are typical of spring weather, favour the germination of aeciospores.

Once infection occurs, it takes between 3 and 4 weeks for telia to develop. They show up as dark spots on the underside of the affected leaf. Once teliospores or basidiospores have infected new buds, the fungus becomes systemic and perennial. Both fungal species can overwinter as systemic mycelium, and G. nitens may also overwinter as teliospores.

Scouting Notes
After leaves have just emerged in early spring, check new primocane growth for deformed, yellow leaves. Look for spindly, clustered new shoots that develop from the crown. Infected canes will have few if any spines.

Search the underside of leaves for waxy, blister-like pustules. Towards the end of May, search the underside of leaves for powdery, bright orange blisters.

During the summer, check the underside of mature leaves for dark brown spots, especially in the lower canopy and toward the middle of the trellis.

Thresholds
None established.

Management Notes

• Plant disease-free black raspberry and blackberry stock.
• Remove infected plants, including the crown and roots, in early spring, before the appearance of bright orange, blister-like aecia.
• Remove nearby wild brambles, which are a source of inoculum.
• Prune out old fruiting canes right after harvest.
• Thin canes and maintain narrow rows to improve air circulation. This helps prevent long periods of leaf wetness that favour infections.
• Control weeds.
• Certain blackberry varieties may exhibit some resistance.