The story of the arrival of poplar rust is almost forgotten and many of those involved long retired or dead. This blog is based on a PowerPoint presentation that I gave to foresters as part of a biosecurity forest pathology and biosecurity training at the turn of the century (I love that phrase).
Infected shelter belt [photo ?]
My first recollection of poplar rust was as a teenager living in the Manawatu. My step-father bought a bundle of bare-rooted semi-deciduous poplar saplings to plant a hedge down one side of our section. This was mid-1972 and these semi-deciduous poplars where new and all the rage for shelter belts at the time. In mid-1973 our hedge got poplar rust.
History of poplars in New Zealand
Poplars were first brought to New Zealand between 1840 and 1850 and these were mainly Lombardy poplars and cottonwoods. Poplars were planted widely across the landscape as shelter belts and homestead plantings.
Mendip Hills Homestead (owned by Andrew William Rutherford), with croquet lawn laid out in front, and poplar trees left background. Photograph taken by Albert Percy Godber in 1917 [Godber Collection, Alexander Turnbull Library]
Between 1850 and 1968 around 150 species, cultivars and hybrids clones had been introduced into New Zealand. Eighty of these arrived between 1957 and 1968. Many of these later imports where by the Soil Conservation Service of the Water and Soil Division of the Ministry of Works.
Use of poplar timber
The 1962 survey of exotic forest in New Zealand found about 8000 acres planted in poplars. These were mostly as shelter and riverbank protection and consisted of the European Lombardy and white poplars which are of low quality timber. But, there were also 500 acres of black poplar and its hybrids which that had been grown specifically for their timber.
This is pretty small scale and poplars for timber had not taken off because there was a lack of good alluvial sites in New Zealand’s State Forest, poor understanding of the trees needs and inappropriate silvicultural practices, and the growing dominance of Pinus radiata as the preferred timber production plantation species.
Local use of poplar timber had been for truck decking, furniture, veneers, boxes, battens and gates. We also imported poplar timber from Canada to make matches. By 1968, there was a growing interest in developing its use in veneers and for paper production.
The growing farm Forestry movement saw the increased planting of poplars in conjunction with agriculture and on better soil types. This was actively encouraged by the New Zealand Forest Service through a funding scheme.
Poplar use in soil conservation
By 1968 400,000 poplars were being planted annually, encouraged by the availability of a soil conservation subsidy, and it was expected that this would quickly reach a million trees a year. It was forecast that over the next 30 years that 32 million poplars and willows would be planted and if managed correctly would provide a significant timber resource.
Poplars in the New Zealand rural landscape [photo FRI Bulletin 124]
At this time, 1968, other than a couple of leaf-spots, there were no poplar diseases in New Zealand. However, and there is always a however, Chris van Kraayenoord, who worked for the Ministry of Works and was known as “Mr Poplar”, was voicing his concern that with this rapid increase in planting and with most planting occurring on “marginal” sites for erosion control there was a very real potential for a disease outbreak. Chris actively discouraged the use of varieties that were known to be susceptible to disease overseas. And, new cultivars known to be disease resistant were brought in to New Zealand to replace the cultivars of “unknown origin” that we were using.
In our modern world of biosecurity alertness the quarantine requirement were, and had been from 1952, for plants stock entering from Europe, North and South America to be grown for one season in the Department of Scientific and Industrial Research’s (DSIR) quarantine facility.
Rust outbreak in Australia
On the 27th of January 1972 American poplar rust disease (caused by Melampsora medusae) was found on cottonwoods (poplars) in the Macdonald River valley 64 km northwest of Sydney. Inspection of the area on the 3rd of February found the disease spread along 16k m of the river valley in cottonwood plantations. Within a few weeks the disease had spread beyond the initial area and by April 1972, it had reached the Queensland and the Victorian borders as well as spreading 400 km inland. This rapid spread was the result of the surface wind and wet cyclonic conditions during this period.
American poplar rust spread in NSW [map Walker, Hartigan, Bertus, 1974]
In the 1972 inspection all poplar species were examined for rust and Lombardy poplars were found to be disease free. But, in February 1973 rust was found on a Lombardy poplar in Sydney. This was the European poplar rust (Melampsora larici-populina
). Like the American poplar rust it spread rapidly so by May 1973 it was found from Sydney to the Victorian border and as far west as Cobram in the Murray River valley.
Poplar rust in New Zealand
Warnings were issued to New Zealand growers in mid 1972 to be on the lookout for poplar rust. on the 21st of March 1973 American rust was confirmed in Kaikohe, Northland. Over the next four days nine further sites were discovered in Northland. Initially the Forest Service attempted to stop the spread by spraying and burning 2000 trees.
Spraying poplar rust, Northland [photo FRI]
Burning infected poplar, Northland [photo FRI}
Also the Forest Disease Control Regulations (1967) were invoked that prohibited the movement of polar material out of the Hokianga and Bay of Islands counties. On the 26 of March it was realised that the disease had spread so far that destroying any more trees was pointless. By the 27 of March all catchment authorities throughout New Zealand had been alerted to be on the lookout for the rust.
Spread of poplar rust in the North Island [map van Kraayenoord, 1973
A survey team, made up of Ministry of Works and Forest Service staff, did a disease survey of Northland and reported its findings on the 28th of March. Its recommendations were:
In view of the present widespread distribution of the rust from Kaitaia to Raglan, at least twenty localities, the distribution of the rust has passed the stage at which the rust can be eradicated.
- Because of the already widespread distribution of the rust in Auckland conservancy and judging by the rapid spread of other rusts and this one in Australia, it is considered that Melampsora will spread to the rest of New Zealand in the near future. It is felt that little would be served by further invoking the regulations at this time.
- It is considered that the propagation of the Italian hybrid poplar I.78 which is the most seriously affected will be phased out. The position regarding the other commonly planted hybrid poplars is not yet clear, but the Yunnenensis, Lombardy and Silver poplars appears to be free of the disease.
- Unless the disease behaves very differently in New Zealand than it has done in Australia and America, pine plantations in New Zealand are not at risk at all. Infection of P.radiata has only been achieved under highly artificial experimental conditions in Canada and only on very young seedlings. No infection of P.radiata has been found in the field anywhere in the world.
By the 16 May rust had been found right down the west coast of the North Island. But, around New Plymouth Lombardy and Robusta poplars had rust and this was not the case in Northland!
The New Plymouth outbreak had been found on the 27th March 1973. A survey on the 29th of March found the disease 16 km from New Plymouth and 50 km a few days later. From 28 March it was official confirmed that this was European poplar rust not American as found in Northland.
By mid May American poplar rust had swept down into the Waikato, across the Bay of Plenty and East Cape to Gisborne. There was one isolated outbreak in the poplar breeding nursery in Palmerston North but that is thought to have originated from disease material sent in for diagnosis. Meanwhile European polar rust swept north into the Waikato, south along the west coast, and east into the Wairarapa and Hawkes Bay.
It was uncertain how these two species would react in New Zealand’s mild climate. There were two major concerns. One, that with a large population of semi-evergreen Lombardy poplars that the urediniospores would over winter on the poplars. This was confirmed the first winter (1973) with viable urediniospore being produced all winter by European poplar rust. This allowed inoculum levels to build up continuously. American poplar rust did not fair so well as the Lombardy poplars were partially resistant and deciduous so there could be no over winter build up of inoculum. Also, these rust have alternative hosts, American poplar rust on larch, Douglas fir, and experimentally on radiata, ponderosa and contorta pine, and European poplar rust on larch and it was unknown how these alternate hosts would be affected.
By the spring and summer of 1973-4, European poplar rust had spread right across the North Island. It crossed Cook Straight and appeared in Nelson in December 1973 and reached Invercargil, in the far south, by May 1974. However, American poplar rust did not spread any further and in fact contracted into a few isolated localities and eventually died out.
The life cycle of these rusts are complicated involving two different host species and five different spore types! This is in their native range it was not known what would happen in New Zealand where the hosts and the rusts are both exotic introductions.
Infected poplar leaf [photo FRI]
During the summer the infected poplar leaves develop lots of pustules producing large quantities of yellow of urediniospores which give the diseased leaves their golden colouration. One infected leaf can produce between 10 to 20 million spores. These spores are are dry and powdery and spread by the wind for great distances to infect other poplars.
Pustules of urediniospores [photo Forest Pathology in New Zealand 20]
Leaves with heavy infections soon die and can fall from the tree with four weeks of infection. In late summer, before leaf fall, the pustules begin to produce a second type of spore which is black, the teliospores. These are not shed but over winter on the fallen leaves. In the spring they germinate to produce the third spore type, the basidiospores which can infect only the alternate conifer host. On the conifer host the fourth and fifth spore types are produced, the pyniospores are first produced then in early summer aeciospores are produced. The aeciospores can only infect poplars and thus continue the life cycle.
As already pointed out the rapid success of European poplar rust in New Zealand was the presence of semi-deciduous poplars which have some leaves all year. This meant that the pustules that produced the yellow urediniospores could over winter on the infected poplar and be ready to infect new foliage in the spring by passing the need for an alternate conifer host.
In New Zealand the presence of rust on conifers has been rarely seen probably due to poor basidiospore dispersal. Infection of larch species was only seen when when they have been grown along side diseased poplars and then only causes some minor leaf fall.
The effect on poplars has been more significant. Due to the high inoculum levels poplars have been subjected to constant defoliation. This does not kill the tree outright but weakens it making it prone to other disease such as silver leaf (Chondrostereum purpureum) and thus prone to wind breakage and toppling. Also repeated defoliation reduced growth rates significantly.
How Did the Disease Get to New Zealand
It is generally accepted that the disease was wind blown from Australia. Meteorological data showed that wind conditions at 3000 metres were suitable between the 28th of February and the 3rd of March. Spores produced in Australia could have been carried to that height by thermal currents and quickly blown to New Zealand. Under ideal conditions it takes only 8 to 10 days from infection to first spore production so that an arrival date of 3 March would allows two generations/ infection cycles to have developed before it was discovered on the 21 of March.
Since the 1970s
Flight path across the Tasman Sea [Close, Moar, Tomlinson, Lowe, 1978]
American poplar rust failed to establish itself in New Zealand and had died out by 1984.
By 1987 a number of new cultivars and hybrids resistant to both rust species had been produced and released commercially. In some cases resistance was specifically for European poplar rust and not so good for American poplar rust.
American poplar rust was detected in Hamilton in 1990 and was thought to be a fresh introduction from Australia and again, died out.
In March 1991 some of the new rust resistant cultivars in Northland where infected by rusts. This rust appeared to be a hybrid between European and American poplar rusts. It failed to overwinter in New Zealand and died out. Again it is thought to have blown in from Australia although it is not known there.
Close RC, Moar NT, Tomlinson AI, Lowe AD 1978. Aerial dispersal of biological material from Australia to New Zealand. International Journal of Biometereology 22: 1-19.
Spiers AG 1990. Melampsora leaf rusts of poplar. Forest Pathology in New Zealand 20: 1-8. Forest Research Institute, Rotorua.
van Kraayenoord CWS 1973. Poplar leaf rust comes to New Zealand. What’s New in Forest Research 7: 1-4. Forest Research Institute, Rotorua.
Walker J, Hartigan D, Bertus AL 1974. Poplar rusts in Australia with comments on potential conifer rusts. European Journal of Forest Pathology 4: 100-118.