Spread of bacterial wilt continues

Press, Volume CXVI, Issue 34046, 9 January 1976, Page 6

Spread of bacterial wilt continues

Bacterial wilt has been found on lucerne in the Rangitikei. Laboratory analysis has confirmed the presence of the disease in a stand close to the Flock House Farm Training Institute near Bulls.

The Ministry of Agriculture and Fisheries says that this finding is further evidence of the increasing incidence of the disease, which causes stunting and a characteristic ‘‘witch’s broom” effect. It can kill infected plants within 18 months. There is no known treatment for affected stands.

The wilt-causing organism was first identified in Mid-Canterbury in 1970, and until fairly recently, was thought to exist only south of the Conway river.

But the disease has now been reported in areas including the Waikato, Hawke’s Bay, the Rangitikei and the RotoruaTaupo regions. However, it has not yet been identified in Marlborough, one of the major sources of lucerne seed.

According to Mr J. D. Currie, the chief advisory officer (plant health) of the Ministry of Agriculture and Fisheries, the latest finding of the disease was made during a nationwide survey of lucerne’s disease and pest status being undertaken by the Ministry and the Department of Scientific and Industrial Research.

Bacterial wilt was only one of several problems currently facing lucerne growers, he said Diseases such as verticillium wilt and crown rot, and insect pests such as stem nematode. sitona, and white fringed weevils had been reported With lucerne playing an increasingly important role on many farms a thorough and up-to-date background of its pest and disease status was vital.

The survey would give an indication of how widespread these problems were and would lay the groundwork for future research priorities. Dr I. C. Harvey, of the Ministry’s plant health diagnostic station at Lincoln,

said that field teams would cover more than 200 paddocks throughout New Zealand.

In each paddock 24 randomly chosen plants were dug up and a detailed disease and pest .analysis made of their root, leaf and stem systems. One hundred insect-net sweeps were made across each paddock and soil samples were taken to find any link between pH levels and plant health status. The survey still far from completed, would be a nationally continuing one and any paddocks where pests or diseases were found to be significant would be monitored in the future to measure changes in the levels of infection or infestation.

Information was being collected in a form suitable for computer recording and the Department of Scientific and Industrial Research and the Ministry hoped to have an initial summary completed by the middle of this year. Whenever farmers were suspicious about the presence of any pest or disease, it was in their best interests to take the suspect plant or insect to the local office or the Ministry for positive identification. Dr C. F. Hill, a scientist at the Ministry of Agriculture’s Mount Albert plant diagnostic station, said that hay and silage making provided many opportunities for the further spread of bacterial wilt as infection entered the plant through abrasions and cuts. Contractors and farmers could slow the spread of the disease by learning to identify its symptoms and taking precautions against contaminating clean stands. However, because bacteria: wilt could be present for some time before showing the characteristic stunting and” witch’s broom symptoms, al! lucerne in the least degree suspect should b« managed aa

though infected, and whenever practicable, strict piant hygiene measures should be followed.

Uninfected stands should be harvested before infected stands and younger stands before older stands.

Machinery moving from one lucerne paddock to another posed a special threat, he said. Balers, mowers and other equipment could easily harbour bacterial wilt. This was especially so in known wiltinfected areas and where the same equipment was used to work a large number of stands. Gear should be thoroughly cleaned and washed with disinfectant before moving to a new paddock. Dr Hill said that 5 per cent Janola or an equivalent chlorine bleach, or 5 per cent formalin, were suitable for this.

He said that rhe spread of wilt was enhanced by wet conditions and crops should only be worked or mowed when dry. Irrigation could also aid in spreading the disease, especially within a stand. Bacteria entered the soil water and infected nearby plants. To limit this, lucerne should not be sprinkle-irrigated to the extent that noticeable horizontal water movement occurred in the soil.

The rate of spread could also be limited by making sure that grazing stock did not move directly from one lucerne stand to another.

The increasing number of identifications of bacterial wilt had to be regarded with concern, said Dr Hill, and every effort made to minimise its spread, at least until wilttolerant varieties could be established.