Wednesday, May 12, 2010

Blackberry Rusts

I have been spending a lot of time lately in the blackberry fields of Henderson County. This is a really great crop to work with and our growers are excellent. It is a relatively new crop for the county, but over the past few years our growers have planted about 87 acres of blackberries. Most of the varieties that we are cultivating are floricane-fruiting varieties developed from the Arkansas breeding program, including Natchez, Navaho and Ouachita.

One of the things we are learning is all of the disease and insect issues that we are going to experience. I have observed two rust diseases on blackberries so far this season and would advise all growers to be scouting for these problems.

Section of Natchez planting with an obvious problem.

The first disease I observed was causing a problem only in Natchez. It is cane and leaf rust. The infected section of plants had some rust problems late last season. The grower has heavily pruned out infected canes and applied products for management.

The causal agent of cane and leaf rust is the fungus Kuehneola uredinis. Cane and leaf rust is first seen in late spring on infect floricanes. The large yellow-orange spores of the pathogen split the bark. At this point, we have not observed any rust on the leaves.

Vibrant yellow rust spores emerging from cracked bark.

Bark cracking in multiple spots on Natchez cane, exposing rust spores.

Pustule of K. uredinis breaking the bark on Natchez.

Control of cane and leaf rust involves cultural and chemical control. The first step is to remove heavily diseased canes and dispose of them in order to reduce inoculum (spores). Highly susceptible varieties or areas with a history of this problem require chemical control. Products used to control cane and leaf rust are copper, myclobutanil (Rally), pyraclostrobin (Cabrio) and pyraclostrobin + boscalid (Pristine).

Next rust problem...

At first glance there is no problem with these Navaho blackberries, right?

Upon closer inspection, there is an obvious orangey problem. The leaves are also slightly yellowish.

Notice the orange pustules on the underside of the leaves. The leaves also appear pale green.

This problem is orange rust on the variety Navaho. Orange Rust is the most important rust problem in the eastern United States and a limiting factor in the northeast. It is a systemic rust, meaning that it lives throughout the infected plant.

Symptoms of orange rust can be seen in the spring as new growth appears. The young shoots are spindly and the new leaves are often mishapen, stunted and pale green to yellowish. The first spot on the leaves will be a subtle black speck on the upper leaf surface that is caused by the mycleium of the pathogen growing in leaf tissues. In a few weeks, the orange rust spores can be seen on the bottom of the leaf surface.

The infected leaves are stunted, have a slightly yellow hue and the ends are curled.

Close up of the orange-blister like aecia and many orange aeciospores [Aecia = Geek talk for a cuplike structure containing the rust spores (aeciospores)].

There are two forms of orange rust. The form on black raspberry is caused by a fungus known as Arthuriomyces peckianus, while the form more common on blackberry is known as Gymnoconia nitens. Orange rust is not known to kill plants, but it causes reduction in vegetative growth and fruit production. Infected plants rarely recover.

Navaho is susceptible to orange rust. Orange rust has not been observed on Natchez or Ouachita and is seldom a problem on Chester or Hull in North Carolina. Orange rust can also be observed on our wild caneberries. Cornell University has a good factsheet on orange rust.

Control of orange rust includes picking resistant varieties, removing heavily infected canes and plants and applying fungicides. Fungicides (Rally, Pristine or Cabrio) are applied in the spring before pustules are formed on the lower leaf surface. Often the wild blackberries are used as indicators of when to spray. Sprays are applied on 10- to 14- day intervals until the mean temperature remains above 77 degrees F.

I do not have specific organic recommendations available for either of these disease, but the use of resistant varieties, removal of diseased plants (including wild brambles if possible) and diligent scouting will do a lot of good. In addition, here is a great ATTRA publication on the Organic Culture of Brambles.

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