The Berry DisPatch

Newsletter of the Wisconsin Berry Growers Association

July, 2002

Orange Rust and Late Leaf Rust on Brambles

By Patricia McManus

UW-Department of Plant Pathology

In recent years berry growers in Wisconsin seem to be producing a greater diversity of brambles. Perhaps it’s a string of mild winters or new marketing opportunities that have spurred an interest beyond the hardy red raspberry. Along with the new plantings of black and purple raspberries and blackberries comes susceptibility to orange rust, a serious disease that does not attack red raspberries. However, another disease, late leaf rust, is common on red and purple raspberries but not black raspberries or blackberries. Of the two, orange rust is much more serious than late leaf rust. In addition to the fungicide recommendations listed below, management includes eliminating wild Rubus hosts in the area and maintaining good air flow through the canopy.

Orange rust. What makes orange rust so bad is that the pathogen (Arthuriomyces peckianus) grows systemically throughout shoots, crowns, and roots. Once a plant has orange rust, it will always have it. Orange rust usually doesn’t kill plants, but it stunts and weakens them so much that they yield poorly and are essentially worthless. New shoots in the spring are weak and spindly with pale, stunted leaves. Within a few weeks, the undersides of leaves are covered with shiny, orange pustules that can be so numerous that they overlap. Leaf infections are localized. However, spores from these pustules give rise to a second type of spore that infects new shoots emerging from the crown of the plant. Shoot infection is systemic, meaning that the fungus becomes established within crowns and roots where it can safely overwinter. It used to be that the only way to manage orange rust was by removing infected plants. However, Nova 40WP (myclobutanil), a systemic fungicide, is now registered on brambles and is effective against orange rust. The label recommends 1.25 to 2.5 ounces per acre, starting at budbreak and continuing at 10- to 14-day intervals depending on the disease(s) to be controlled and environmental conditions. For orange rust, there are two important periods where Nova is most effective. The first is in the spring when orange spore-filled pustules are first detected. The fungicide will protect uninfected leaves and therefore reduce overall spore production. The second critical period is during late summer and early fall as temperatures become mild. The fungus grows best at temperatures of 43 to 72 degrees F with rain. Thus, it is essentially dormant during hot, dry weather, and Nova would not be needed under those conditions. However, if summer weather is cool and wet, and new shoots are forming at the crown, then Nova should be used.

Late leaf rust is so named because it develops later in the summer than orange rust. The pathogen Pucciniastrum americanum does not become systemic, and therefore late leaf rust is not as serious as orange rust. However, fall-fruiting red raspberries such as Heritage can sometimes suffer losses from leaf drop and fruit infection. The earliest symptoms are faded, chlorotic or yellowish spots on the upper leaf surfaces in late summer. Later the undersides of leaves appear dusted with rusty-colored spores. This is quite different from the bright, waxy appearance of orange rust pustules. Spore masses can also appear on petioles, canes, and on one or more druplets of a fruit. The biology and life cycle of P. americanum are poorly understood. In some regions, the fungus overwinters on white spruce trees. However, the disease occurs in the apparent absence of spruce trees, so the alternate host is probably not required for the pathogen to complete its life cycle. Nova is the only fungicide effective against late leaf rust. It should be used as soon as symptoms are noticed to prevent further disease development and crop loss.