Witches' broom disease of lucerne : (1) witches' broom of lucerne in relation to environment : (2) the identity of witches' broom of lucerne and big bud of tomato in Australia
Creators: name: Helms, K
Publisher: name: University Of Tasmania
Year: 2023

description: (1) Witches' broom of lucerne in relation to environment Masking of symptoms of witches' broom infected plants occurred annually in the spring growth in the Lachlan Valley, New South Wales. Symptom expression was shown to be influenced by frequency of cutting and availability of water, and it appeared to be influenced by day length, light intensity and temperature, either directly or indirectly through the effect of winter dormancy on plant growth. Shoots from plants in which symptoms were masked contained virus particles. The maximum disease incidence of witches' broom was observed in summer or autumn between February and June, the time being influenced by rainfall. At the time of maximum disease incidence, severe symptoms were associated with hot, dry conditions and mild symptoms with high rainfalls. Variability of symptoms at any one time was partly due to distinct types (strains) of field infected plants. Clonal cuttings developed from two distinctive field infected plants maintained their characteristic features in each of L. cultural treatments, although symptom expression varied within clones, between treatments. The seasonal pattern of the quantitative data obtained was essentially the same for each treatment. The height and dry weight of regrowth of infected plants was depressed relative to the controls, whereas the water content tended to be increased. Infected plants produced normal, slightly abnormal or virescent flowers which were fewer in number than those of healthy plants. Disease incidence and rate of infection tended to be greater in old, thin stands than in young stands where the plant density was high. Disease incidence was also greater on river terraces than on river flats. These aspects of the disease are discussed. The maximum observed life span of an infected plant was 33 months in the field and 54 months in the greenhouse. In two experiments, in 10 year old stands, the number of lucerne plants was reduced by approximately 12 per cent in 13 months and Witches' Broom was considered to be the most important single factor causing mortality of plants. A high mortality of infected plants was associated with frequent cutting and more mortalities occurred on river terraces than on river flats. Data on the disease from experimental plots in Queensland are compared with those from New South Wales, and the economic importance of the disease in Australia is discussed. (2) The identity of witches' broom of lucerne and big bud of tomato in Australia The symptoms of witches' broom virus disease of lucerne on 8 new host species are described. Two naturally occurring strains of the disease produced distinctive symptoms on all hosts to which they were transmitted. On 14 host species the more severe of the two strains and similar naturally occurring strains, produced symptoms that were indistinguishable from those of big bud of tomato. Witches' broom and big bud are considered to be caused by the same virus because they are transmitted by the same vector and they have a similar host range on which they produce similar symptoms. Symptoms of Witches' broom of lucerne in Australia are compared with symptoms of witches' broom of'lucerne in America and some other yellows viruses.