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/
Plants may be infected at any stage of growth provided environmental conditions are favourable. Infection of seedlings occasionally occurs; however, the first infections are normally detected in plants bearing three to five leaves. Initial stages of infection are confined to the host root system and base plate (Scott, 1956). The first above-ground symptoms of infection include a yellowing of leaves beginning at the tips and progressing downward. A gradual decline in the plant continues for some days or weeks and in the case of young plants may constitute a rapid wilt and collapse of aerial parts (Walker, 1924). Ultimately the entire plant is killed. On underground parts the fungus itself is visible as superficial, fluffy white mycelium. The roots are gradually destroyed and the fungus causes a soft, watery decay of the bulb commencing at the base plate (Walker, 1924). Black spherical sclerotia, normally 200-500 µm diameter, are formed on the bulb base and within decaying root and stem tissue. A larger sclerotial form has also been reported to appear as a stromatic mantle among smaller sclerotia (Metcalf et al., 1997).
Above-ground symptoms are not normally evident until the pathogen has colonized and partially rotted the stem and leaf sheaths. Roots frequently extend horizontally, providing a direct path for mycelial growth to nearby plants. Infected plants therefore tend to occur in clusters from a few up to 40 or more adjacent plants (Crowe and Hall, 1980a).
Solarization has limited usefulness for cooler climates. Some control has been achieved under experimental conditions using this method (Porter and Merriman, 1985). Soil is covered in black plastic sheets to conserve solar radiation, thus raising the temperature to levels lethal to sclerotia or more favourable for antagonistic microbes. A review of the use of soil solarization for the control of soilborne pests has been compiled by Katan (1981).
Prolonged periods of waterlogging have been associated with decline in sclerotial populations. In one study, nylon-covered PVC canisters each filled with 1000 sclerotia were buried at a depth of 10 cm in field soil which was subjected to flooding from April to October in 1992, 1993, both years, or not flooded (control). After 1 month's burial prior to flooding, 920 (mean) sclerotia per canister were recovered. In the treatment flooded in both years, the number of viable sclerotia was eight in October 1992 (survival in unflooded control: 455), and zero in October 1993 (survival in unflooded control: 422). However, in the treatment flooded only in the 1993 season, fewer viable sclerotia were recovered in the unflooded control (422) than in the treatment (670) (Crowe, 1995b). These results could reflect a greater predisposition for sclerotial decline in the first 12 months after formation - other workers have reported extensive sclerotial decay in this period (Alexander and Stewart, 1994) - or the results may be due to seasonal difference and random variation.
Two types of resistance mechanisms to white rot have been identified. The first, non-stimulatory resistance, occurs where the stimulatory capacity of the root exudates is too weak to induce the sclerotia to germinate and attack the roots (Rahe, 1981). The second, tissue resistance, does not influence the number of infection sites but impedes the spread of the infection in the plant (Rahe, 1981; Brix and Zinkernagel, 1992). No resistant cultivars are currently commercially available.
Sclerotial Germination Stimulants
These compounds are imitations of the exudates of Allium roots, and when applied to the soil they trigger sclerotia to germinate. In the absence of a suitable host, the hyphae from the germinated sclerotium die, thus reducing the S. cepivorum inoculum density. The most effective compound for this purpose developed so far has been diallyl-disulphide, which has been used to induce significant reductions in sclerotial numbers at certain concentrations.
It is important to apply sclerotial germination stimulants when soil conditions are most conducive to sclerotial germination. Soil needs to be wetter than -300 millibars, and warmer than 9°C. At a matric potential of 0 and temperature of 27°C, 100% of buried sclerotia germinated, and a clear relationship between sclerotium germination and soil moisture content and temperature was established (Crowe and Hall, 1980b). Commercially acceptable levels of disease control have recently been achieved using sclerotial germination stimulants in industrial-scale trials (Dennis, 1997, 1998).
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:
On a worldwide basis, Allium white rot is probably the most serious threat to Allium crop production of any disease. It is certainly among the most important threats in most onion-producing regions, sometimes second to neck rot (Botrytis allii), leaf blight (Botrytis squamosa) or downy mildew (Peronospora destructor). It is present in almost all Allium-producing regions of the world. While some regions have been able to continue production despite infestation, the disease has never been completely successfully managed anywhere. In a number of regions the disease has been responsible for the complete collapse of the Allium production industry.