Beginner

Scientific Name: Pucciniastrum americanum

Identification

• Infects red and purple raspberries; fall-bearing cultivars are most susceptible.
• Infections occur on mature leaves on actively growing plants.  
• Infected leaves develop small, yellowish, chlorotic spots on their upper surface
• Infected leaves eventually turn brown.
• Orange-yellow pustules develop on the underside of infected leaves, flowers, petioles and individual drupelets of fruit.
• In severe cases, infected leaves may die and drop prematurely resulting in bare canes

Often Confused With
Orange Rust of Blackberry

Period of Activity
White spruce is the alternate host for this disease. Infected white spruce produce  air-borne spores between mid-June and early July that can  travel for many miles.  The first orange-yellow pustules show on the underside of mature raspberry leaves in the beginning of July and these first lesions produce new infections on leaves and fruit throughout the growing season. The most serious symptoms are noticed in late summer. Fruit infections can be quite serious on susceptible fall-bearing cultivars. The disease is most severe at temperatures between 18°C and 26°C. At 20°C, it only takes about 6 hours for urediniospores to infect a leaf.

Scouting Notes
Examine the underside of mature raspberry leaves and check for powdery, orange-yellow pustules. Later in the season, examine the fruit and look for powdery, orange-yellow pustules on individual drupelets.

Thresholds
None established.  

Advanced

Scientific Name: Pucciniastrum americanum

Identification
Late leaf rust affects red and purple raspberries. The disease has the greatest economic impact on fall-bearing cultivars which produce fruit later in the season.  Summer-bearing cultivars are often harvested before fruit infections occur.

Mature leaves on actively growing plants develop small, yellowish, chlorotic spots on their upper surface. These spots are usually inconspicuous, especially in mild cases of the disease. Infected leaves eventually turn brown.

Severe late leaf rust causes premature defoliation and fruit infection. By July the underside of affected leaves will develop pustules of powdery, orange-yellow urediospores. These orange-yellow pustules may also form on flowers, petioles and individual drupelets of fruit, which are susceptible to late leaf rust infection at all stages of their development. This makes the fruit unsightly and unmarketable. When the disease is severe, affected leaves may die and drop prematurely resulting in bare canes earlier in the season. This in turn predisposes the plants to winter injury.

Often Confused With

Orange Rust of Blackberry
Both late leaf rust and orange rust are fungal diseases, however, late leaf rust affects red and purple raspberries, while orange rust affects blackberries, black raspberries, and purple raspberries. Orange rust causes waxy blisters filled with bright orange spores on the underside of infected leaves. Late leaf rust produces spores which are orange-yellow and powdery. In addition, orange rust is a systemic, perennial disease, while late leaf rust is not systemic.

Biology
Late leaf rust disease is caused by the fungal pathogen Pucciniastrum americanum. Unlike orange rust, the disease is not systemic. The life cycle of this disease is very complex with many types of spores. The fungus requires two different host species to complete its lifecycle. In this case, the alternate host, white spruce (Picea canadiensis) produces all spore stages (aeciospores, urediospores, teliospores, and basidiospores). Uredinia, telia and basidia develop on red and purple raspberries. Between the middle of June and the beginning of July, aecia form on white spruce needles and cone scales and release aeciospores.

These are wind-dispersed and infect mature raspberry leaves. In the case of summer-bearing cultivars, uredinia form on the underside of leaves of fruiting laterals and on flower calyces in early July. The wind-dispersed urediniospores will then re-infect plants throughout the remainder of the growing season. Infection occurs through stomata on the lower leaf surface. Later in July, uredinia will also form on individual fruit drupelets and on the lower leaves of primocanes and fruiting canes. During harvest, urediniospores can be dispersed mechanically by people or machines.

In fall, rust infection rates slow due to cooler temperatures. Raspberry leaves infected with urediniospores will now form telia which will overwinter and form teliospores in the spring.

Between mid-May and early July teliospores on raspberries germinate to form basidia and basidiospores. Teliospore germination requires moist conditions and the rainy periods of early spring are ideal for this process to occur. Basidiospores are released at a time when white spruce is growing rapidly. The basidiospores infect buds and spruce needles
Late leaf rust seems to infect raspberries even if white spruce is not growing nearby. This suggests that the fungus can overwinter in raspberry canes and re-infect raspberries the following year.

Period of Activity
White spruce is the alternate host for this disease. Infected white spruce produce  air-borne spores between mid-June and early July that can  travel for many miles.  The first orange-yellow pustules show on the underside of mature raspberry leaves in the beginning of July and these first lesions produce new infections on leaves and fruit throughout the growing season. The most serious symptoms are noticed in late summer. Fruit infections can be quite serious on susceptible fall-bearing cultivars. The disease is most severe at temperatures between 18°C and 26°C. At 20°C, it only takes about 6 hours for urediniospores to infect a leaf.

Scouting Notes
Examine the underside of mature raspberry leaves and check for powdery, orange-yellow pustules. Later in the season, examine the fruit and look for powdery, orange-yellow pustules on individual drupelets.

Thresholds
None established.  

Management Notes

• Heritage and Festival are very susceptible. Nova is highly resistant to this disease.
• White spruce is a host for this disease. Removal of white spruce in the vicinity of the planting may help reduce the potential for disease on raspberry.
• Use management practices that encourage plants to dry quickly after rain. Keep rows narrow and control weeds to improve drying in the row.  Late leaf rust thrives under conditions of high humidity. Frequent rainfall and excessive sprinkler irrigation increase infection rates.
• Remove old flowering canes and infected primocanes at the end of the growing season to reduce or eliminate infection in the following year.
• Start new plantings with disease-free stock.