NZTCA - Research - Phytophthora Resistant Chestnut Rootstocks

EVALUATION OF FIVE
PHYTOPHTHORA RESISTANT CHESTNUT ROOTSTOCKS
FINAL REPORT AUGUST 2005

PHYTOPHTHORA COLLAR ROT DISEASE IN NEW ZEALAND.

“A widespread disease of apples & chestnuts best controlled by good orchard management”

BY Harvey C. Smith M. AGRI. SC NZ, Ph.D CAMBRIDGE ( Previous Officer in charge Plant Diseases Divn, Lincoln & Director Crop Research Divn. D.S.I.R. NZ )

Phytophthora collar rot has been recognised in New Zealand as a cause of deaths of orchard trees for only 50 years when I wrote an article in the NZ Orchardist ( Smith H.C. 1950 Collar-rot of Apples and Gooseberries. Orchardist N.Z. Vol.23, No 11 ,p. 11-14) describing my discovery that the disease previously called sour sap or root-rot in apples , gooseberries was not just caused by excessive water in the soils but was caused by the active rotting of the large crown roots and the bark just below and near the soil surface .( I also made the first discovery of this disease in the U.K. in Essex in 1953 while at Cambridge. See Plant pathology Vol 2,No. 3, p. 85-86) The fungus was isolated and found to be actively infective to the bark tissue of a wide range of orchard and ornamental trees and shrubs in a wide area in all districts of NZ.

After several years of study several different species of fungus were identified with differing degrees of severity to the different orchard and ornamentals . The most commonly associated condition of this collar rot disease was excessively wet soils. This was consistent with the nature of the different species of Phytophthora found which all produced masses of fruiting spores which could swim in the soil water and actively invade the actively growing plant roots and when the plant was kept in continuous wet conditions it was deprived of oxygen and the plant were often killed by the ring-barking action of the fungus at the collar of the trees.

Invariably the first symptoms of the disease on the affected plants was yellowing of the foliage of the branches often on only one side of the tree. If the wet soil conditions continued the roots and the bark continued to be attacked and the leaves became red and then brown before dying and falling prematurely.

In the 1950's I supervised a number of field trials in cooperation with the DEPARTMENT OF AGRICULTURE in apple orchards in Nelson and also on other crops to try to find chemicals which would slow down the disease and give some degree of control.

These trials were all unsuccessful. The cost of the treatments far outweighed the very small degree of control achieved.

The main decline of the disease in apple orchards occurred through the almost universal adoption of grassing down of the orchards through-out New Zealand. It is now apparent that the elimination of intensive cultivation on the clay soils of the Moutere hills, the Huapai area and the Loburn areas; with consequent loss of fertilility and soil structure was the main cause of the more severe effect of collar-rot (Phytophthora) in the old apple orchards. Grassing down gradually builds up the soil organic matter and this increases the bacterial and other microbes which control the Phytophthora fungi in the soil.

Chemical control of Phytophthora by phosphoric acid has been advocated by some North Island Chestnut experts but there has not been an adequate evaluation of the alternative methods of control using good choice of organic amendments (e,g, grassing down with frequent mowing and mulches, and high rates of phosphate fertiliser application and improved soil drainage and carefully applied irrigation only at the most active growing periods of the chestnut trees.

The other main suggestions for control by the Northern experts has been the use of soil tests for Phytophythora presence and the emphasis on buying trees from Nurseries which have been tested for Phytophthora presence. My view ( after extensive experience of soil survey for Phytophthora, over many years) is that the causal Phytophthora species are so wide spread in NZ that if the disease is not already present it will inevitably arrive either by air borne sporangia or in soil particles or other organic matter. The orchards take a long period to establish and come into full production. Grassing down is the first initial action for chestnut orchardists who have planted on very poorly drained clay soils , or have poor drainage ;or over irrigate their land before full leaf growth and maximum speed of growth. From years of experience as an orchardist and nurseryman with a wide range of several tree crops:
I am absolutely convinced that Phytophthora collar rot will not be of any economic significance if proper grassing down is established; good free drained soils are selected; careful irrigation is applied; good spacing of trees; generous supply of lime and phosphate fertiliser applied; and the trees are not suffering from incompatibility of rootstock (which also encourages collar-rot).

The incidence of Phytophthora collar-rot disease is a direct reflection of the level of quality of the orchard management.
It is futile to expect that it will be economically and adequately controlled by :
soil tests ! , chemicals ! , choice of nursery !

PHYTOPHTHORA RESISTANT ROOTSTOCKS.

There is no doubt that there is a degree of difference in the genetic resistance to collar-rot infection . This has been known for years in other crops such as strawberries; apples; chestnuts; citrus ; that some varieties are highly susceptible while other varieties show a very useful level of resistance to the disease.
It is also true that although genetic resistance has not been successful in giving complete control the use of more resistant varieties or of partially resistant or tolerant varieties is a very valuable adjunct to improved management practices.
The combination of improved management combined with more tolerant varieties of the crop has certainly given the best and most permanent control.
The project to select for Phytophthora resistance in chestnut root stocks may not be spectacular but any improvement will be a major importance in combination with improved management.

Final Report, August 2005
AGMARDT Grant No. 9917

Evaluation of Five Phytophthora Resistant Chestnut Rootstocks

General Historical Information
Phytophthora is associated with the sickness of crops world-wide, and at least 10 species are involved. In New Zealand we have at least 4 of these effecting different crops. Phytophthora is called variously as collar-rot, bark canker, and root rot.
Species associated to different crops are: P.cactorum with apples, P citrophthora with citrus, P cinnimomi with avocados and pinus radiata, and P citrophthora (previously identified as P cactorum) with hops. The two involved with chestnuts that we know of in New Zealand are P cinnimomi andP cactorum.
Chestnuts in New Zealand have been very susceptible to root rot problems over much of the country. Many trees dying and sick which upsets the uniform layout of the orchard, and can make economics shaky. The worst aspect is that trees may die after a year or two, but can continue dying even when older. So you don't know how many trees you are going to loose.

Conditions Favourable for Phytophthora
High soil moisture and low oxygen levels are usually associated with root
rot problems. So that all have similar amelioration measures such as:
improving to good drainage, applying phosphonic acid, optimum growing
temperatures, planting on humps not hollows, choosing good friable soil,
avoiding root damage, and ozone injection.
Improving soil drainage and draining surface water away quickly from
around the trunks will give the trees a better chance.
With other crops such as apples, resistant rootstocks have been a means of controlling root rot problems on infected soils. That is why we have been looking at the possibility of resistant rootstocks for chestnuts.

New Zealand trials

Dr Harvey Smith in 1998 tested three varieties and five rootstocks with
artificial inoculation of both P cinnimomi and P cactorum. Four of the
rootstocks showed a degree of tolerance to both species. One seemed more
tolerant of P cactorum than P cinnimomi.
Dr David Klinac of Hort research tested C saliva and C crenata, plus
hybrids of the two by artificial methods. This included several thousand
seedlings of 1002, 1005, and 1015, plus a collection of several other types
from around the country.
The tests involved waterlogging alone (in the absence of phytophthora,
and also introducing increasing levels of Phytophthora cinnimomi by
inoculation.

Results:
a. No seed-line was totally resistant; all could eventually be killed by increasing the concentration. Therefore the resistant types should possibly be called tolerant not resistant. Although the true definition of resistant does really apply, as it doesn't imply immunity.
b. Water-logging alone also killed plants eventually, but at a slower rate than phytophthora.
c. There was a range of tolerance in every seed-line.
d. C crenata lines were more tolerant than C sativa lines.
e. Rootstock resistance was more important than a susceptible scion on top.
f. The scion/rootstock combination of C crenata 902 gave best results.

The rootstocks numbers 1,2,3,4,5,6, came from the best lines of C sativa from this work. This was important because they are compatible with the standard varieties 1002, 1005, and 1015.

This series of Trials

These rootstocks have been tested and selected by artificial inoculation previously, (as stated above) and this series of trials was to field test them to confirm or otherwise their resistance. Although we state individual phytophthora species, it must be understood that in the field, trees are usually confronted by a complex of diseases and species to deal with. That is why field testing is so important.

Results in Brief of the Field Trials:
Growth and results have been quite variable at different sites. However, there is reasonable evidence that No4 rootstock has quite good resistance. 902 are very resistant. No. 1 and No. 3 with further testing could prove resistant. The others cannot be recommended. Along with these results it must be realised that any living tree can be subject to conditions it cannot withstand. Improving drainage is the key to solving root rot problems. Also resistance is not immunity, so that there are levels of resistance, and conditions can get so severe that anything will die.

A subsequent trial using organic matter, lime, Foli-R-Fos, and susceptible and resistant rootstocks in all combinations, showed no advantage from any treatment other than the resistant rootstock used 902. Susceptible rootstocks were all dead within two seasons.

Trial Sites for the selections No. 1,2,3,4,5,6.

Canterbury:
No. 4 was clearly the best selection at this site, with only one tree suffering from being mown off by a tractor. All other selections had a tree die of Phytophthora, along with some weak trees. These trees were closely planted making a condensed trial. It was subject to artificial flooding to ensure infection. No. 3 also showed some promise.

Motueka Valley:
There seemed to be a very severe area at this trial, in the middle of the trial, where all rootstocks began dying. Trees were subject to very strong twitch right up to their trunks being a non-spray property. This restricted growth considerably. Unfortunately we didn 't have No. 4 at this site due to shortage of plant material. Of the selections here, No. 3 performed the best. The trial trees were planted at sites where the previous tree had died of root rot.

Horowhenua:
This was thought of as the most severely infected site at the beginning of the trials. However, both Motueka Valley and Canterbury seemed as bad by 2005 at the end of recordings. Previously 902 rootstock trees had been the only ones to survived here in this orchard. This was true again when we had the treatment trial there (as mentioned above), with only 902 trees alive after 2 years. All the trial trees were planted at sites where the original chestnuts had died.
All of the own rooted trees obviously established best to start with. However, now the grafted trees have rooted with the selections own roots, they are now starting to catch up in size. There are good observable differences at this site for the various rootstocks. 902 's are extremely healthy, as are No.4's. No.1 is looking promising, but No. 6, although quite big, has unhealthy looking leaves. The rest are poor.

Waikato:
For some reason, trees had to be moved from this site after two seasons, and were put temperately at Rukahia Research Station in big tubs. When subsequently moved to a Northland property a year later, they all died; presumably from transplanting shock and bad treatment.
All No. 1, No. 4. and No. 6 own rooted trees were doing well before being moved, but disease severity appeared low when the trees were moved. Although two had died and one more was sick looking. I had the feeling that root rot at this site was alleviated by the auger used at planting going very deep, and improving drainage.
Trees were planted close together in a compact trial.

Species Analysis:
Dr Harvey Smith, a disease specialist, is testing the soil and root samples from the various sites to identify the specific phytophthora species involved. There has been a suggestion that the balance of species are different in the North and South Islands. However, our most recent observations in the field suggest no difference in reaction to root rot at the various sites. These laboratory tests have not been completed yet, but will be added as they become available.

902 rootstock grown from seed showed the best resistance from this work, but is incompatible with 1005, 1002 and 1015. However it is a good nut cultivar on its own account. Of the rootstock selections that are compatible with 1002, 1005 and 1015, No. 4 seems the best. Some of the others may prove to be useful after further work, especially No. 3. It was difficult to tell exactly, but I think No. 4 's resistance is almost as good as 902 's.
So that No. 4 seems the best rootstock to go with if you are to try growing the standard cultivars on resistant rootstocks at present.
Initially this would have to be done by rooting cuttings, stool beds, or grafting. However, it would be a benefit to try growing No. 4 seedlings to see if they are as resistant as the parent tree, as with 902. Deeply planted grafted plants root satisfactorily after 3 years, if tied with a copper wire above the graft. They may root without these ties.
Best practice at present seems to me to be from rooted cuttings of No. 4.

Thanks are due the triallists for putting up with these trials under difficult conditions, and to AGMARDT and NZ Tree Crops Association for financial support.

Trial Leader: Roy Hart
For the NZ Tree Crops Association.

Photos of Trees at different Sites:

Sheet la: of the Canterbury site includes 2 individual healthy trees and a general view showing X's where original trial trees have died.

Sheet Ib: show general views of the Canterbury trial in winter with some trees numbered.

Sheet 2a: showing sick older trees in the orchard at the Horowhenua site.

Sheet 2b: Showing trial trees at Horowhenua site with numbers of the selections.

Sheet 3a: Also the Horowhenua site with best individual trial trees.

Sheet 3b: Horowhenua site showing some of the sick leaf symptoms.

Sheet 4a: Waikato site with individual trees, along with the trees that were put into the tubs.

Sheet 4b: General views of the Waikato site with tree numbers marked.

Back cover: shows a general view of the Nelson site with small trees planted in sites that had dead trees from the original orchard planting. Individual trees are the reasonably healthy ones at this site, although much smaller than at other sites.