If left untreated it will lead to blossom and fruit damage and defoliation, the latter reducing next year’s yields, ADAS berry fruit specialist Janet Allen has warned.

Allen said the disease has become more prevalent in recent years, probably due to climate change. Milder winters and wetter summers and autumns favour its development and spread. Most varieties are susceptible to infection, particularly, Polka, Tulameen, Glen Ample and Joan Squire.

Raspberry yellow rust is unusual in having just one host. Its spores overwinter on the new cane of summer-fruiting varieties and the old cane stubs of autumn-fruiting types, Allen explained.

In the spring, new spores infect the lower leaves of both types and, unless it is well controlled, the disease will spread out on the plants and onto the flower stalks, sepals and fruit.

Outdoor and polytunnel raspberries are equally susceptible to yellow rust, Allen added. Conditions in tunnels, including high temperatures and relative humidity and lack of air movement due to the dense crop canopy, are ideal for spore multiplication and spread.

She advised growers to apply a fungicide at the first sign of attack, displayed as slightly raised, bright orange spots on the tops of the leaves. Crops should be regularly monitored for infection.

Fortunately, growers have a good armoury of fungicides with activity against the disease, she said. They include Amistar, Corbel, Systhane, Folicur and Signum, which can be used on outdoor or protected crops or both.

“If infection is serious, one or two pre-harvest sprays and two or three post-harvest will be necessary,” Allen recommended.

Strong winds at times during May have caused damage to outdoor summer-fruiting raspberries, particularly those on less-sheltered sites, she reported.

Octavia was the hardest hit, with the older foliage turning yellow and necrotic, and even dying. In addition, in some cases the lower bud break had been poor and only the laterals at the tops of the fruiting cane were growing well.