One or more of the features that are needed to show you the maps functionality are not available in the web browser that you are using.
Please consider upgrading your browser to the latest version or installing a new browser.
More information about modern web browsers can be found at http://browsehappy.com/
Infected plants usually develop galls (tumour-like swellings) on roots and stems below ground or at the crown of the plant, hence the name crown gall. A condition known as hairy root (excessive proliferation of roots) is frequently a companion with crown gall on plants such as apple; the symptom is often referred to as non-infectious hairy root because of the difficulty of isolating pathogenic Rhizobium from hairy root tissues. Furthermore, when apple trees exhibiting hairy root at planting time were grown for 4-5 years in the field and lifted to examine root development, the root systems were normal in appearance.
Galls also occur on lateral roots and above ground on stems, canes and vines. On some woody species, such as cottonwood and willow, galls are common on stems above ground. Types of plants affected include shrubs, ornamental flowering trees, fruit trees (temperate to tropical), tubers, vines, tobacco, annuals, gymnosperms, etc. In temperate zones, symptoms appear with the onset of warm temperatures in May or June and can increase rapidly in size and number. Galls first appear as small, callus-like outgrowths within 2-4 weeks of infection when temperatures are at or above 20°C. At first the galls cannot be distinguished from wound callus, but galls usually develop more rapidly than callus, producing white to tan-coloured, more-or-less spherical galls. Gall texture varies from soft and spongy in the early period of formation to hard as they age, depending largely upon the content of disorganized vascular elements. Gall surfaces typically are not covered by an epidermis and can vary in colour. Below-ground galls are initially whitish, changing to tan-brown and becoming necrotic with age; aerial galls on herbaceous plants can be green, depending upon chloroplast content. Aerial galls on caneberries often become necrotic, black and dry during the winter in temperate zones. In the spring, new galls develop adjacent to the old tumour site. Above-ground galls on grapevines, blueberry stems and brambles are usually elongate, erumpent ridges of tissue bursting through the outer stem tissues. Erumpent gall tissue may cause the stems to split and lose moisture, resulting in small, low quality fruit. Galled plants may be stunted and exhibit poorly developed root systems, thus making them more susceptible to adverse environmental conditions such as winter injury and blow-over caused by heavy winds.
Small galls require careful diagnosis because they may be confused with excessive wound callus or with galls induced by nematodes or insects. Isolation is often required to confirm the diagnosis (Moore et al., 1988). Pathogenic Rhizobium strains can be isolated from galls, vascular fluids (mainly grape), and above- and below-ground surfaces of symptomless plants. However, non-pathogens are typically prevalent in these same tissues and can reach high population levels. Large populations of these non-pathogenic agrobacteria are especially troublesome in galls on apple, blueberry and grapevines. They can comprise >99% of the Rhizobium population in the gall, thus making it difficult to find the pathogen and fulfil Koch's postulates. Unusual crown-gall-like symptoms on blueberry have been diagnosed as caused by Rhizobium strains isolated from the galls (LW Moore, Oregon State University, USA, personal communication, 1996). This diagnosis would probably not have been confirmed without the use of DNA probes as very few pathogenic isolates were present in the Rhizobium population.
Sanitation and Cultural Practices
Disease management requires utilization of good sanitation and cultural practices. These include discarding all nursery stock showing symptoms as soon as harvested to avoid contamination of healthy plants; choosing where possible a rootstock that is less susceptible; avoiding planting sites with heavy infestations of root-attacking insects and nematodes; dipping pruning shears in disinfectants; and adopting management practices that minimize wounding. Avoid planting sites where galled plants were grown within the last 4-5 years. Apply irrigation water from wells where possible.
Chemical Control
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any
specific chemical control recommendations. For further information, we recommend you visit the following resources:
Accounts of disease loss caused by crown gall are at best estimates as detailed, accurate surveys are rarely conducted, nor are there good survey instruments available for disease forecast and impact. Plants with crown gall disease are primarily a problem for nurserymen who grow woody plants and shrubs for landscapes and fruit production. Losses in the USA amount to millions of dollars annually from the culling of diseased nursery trees. Vigour and production also can be reduced for landscape plants such as rose and poplar and for fruit trees, grapevines, caneberries and chrysanthemums (Moore, 1980; Schroth et al., 1988; Misic et al., 1990; Nesme et al., 1990).
Plants are damaged most by crown gall when they become infected the first year after out-planting. Severely galled young plants are weakened, stunted, unproductive and occasionally die due to girdling and/or development of an inferior root system (Htay and Kerr, 1974). Contradictions abound in the literature, however, about the injurious effects of crown gall on plants. The reports range from benign (or cosmetic), to debilitating, to death-dealing. However, rose growers in New Zealand (DW Dye, MARC, Auckland, personal communication, 1979) and a Euonymous grower in Missouri, USA (D Millikan, University of Missouri, USA, personal communication, 1978) were again able to grow these plants after they began using the biological control agent, R. rhizogenes K84. Nurserymen in Oregon, USA estimate conservatively that use of K84 to prevent crown gall reduces culling of diseased trees and conservatively saves them one million US dollars annually.
