Host Plant Genetics Discussion

Potato Late Blight On-line Workshop
Potato Host Plant Genetics
Discussion Forum

Hello, I am Paul Tooley, the moderator of the host plant genetics folder.

Please feel free to post messages and contribute discussion relating to either traditional and/or molecular approaches to developing potatoes with durable resistance to late blight. Most agree that resistance would represent perhaps the most viable and cost-effective solution to the late blight problem, over the long run. Traditional approaches have met with some success so far, but the pathogen has proven to be a changeable foe, often causing resistant cultivars to succumb to new races. Horizontal resistance may be the key, but there are differences of opinion on how best to breed for such resistance- also how and where to perform the most meaningful type of screening against the most virulent races.

Molecular approaches also have shown promise, but some of the newer technologies have not been "field tested" sufficiently, or may meet with a certain amount of lack of grower acceptance when finally available commercially. Progress may be impeded by the difficulty of manipulating the host genetically and performing transformation and other molecular methods.

Feel free to post messages and comment on any of the topics covered in the above two areas and listed in the discussions, and offer suggestions for the most effective ways to develop potato cultivars with durable blight resistance for the next millenium. I'll be monitoring the discussions in this area as the workshop progresses, and bringing specific areas of interest to the forefront for further discussion.Thanks for participating.

Traditional Host Plant Resistance

Session Summary

1. Continue to develop collaborative efforts among plant breeders and plant pathologists to establish durable late blight resistance in commercial potato cultivars.

Collaboration among scientists is the key to increasing durable resistance in potato cultivars. Understanding the mechanisms of durable resistance, identifying sources of durable resistance in wild germplasm, exchanging existing breeding lines, adopting standard evaluation techniques and utilizing a diversity of breeding approaches all are needed in expediting this effort.

2. Use existing cultivars with moderate levels of late blight resistance in concert with chemical and cultural tactics.

Considerable time and effort have already been devoted to developing potato cultivars with processing and storage characteristics desired by the industry. Some of these clones have moderate levels of late blight resistance. It is urgent that these lines be identified and utilized to the maximum extent possible via Integrated Pest Management practices. Late maturity currently associated with late blight resistance is not an acceptable trait to the industry at this time, but standards for commercial cultivars could change.

3. Intensify selection of germplasm having both foliage and tuber resistance.

Tuber infection at the end of an epiphytotic is a primary reason for crop loss in the field and in storage, and possibly is related to disease spread the following year. To date, resistance in foliage has been the primary focus in potato breeding efforts. However, late blight resistance in foliage and tubers is not necessarily related. Improving tuber resistance would help prevent direct losses and the potential of tuberborne inoculum. The mechanisms and appropriate assessment of tuber resistance need emphasis at this time. Higher levels of tuber resistance might be required if clones with only moderate levels of foliage resistance are utilized.

4. Coordinate an annual multisite field trial for evaluating germplasm for late blight resistance.

An international field evaluation trial utilizing standard evaluation techniques on an annual basis at test sites throughout Canada, Mexico and the United States is a priority. A carefully chosen set of indicator cultivars to serve as a reference of disease severity should be included so comparisons between sites and years can be made. The trial would become a routine part of the screening process for cultivar development in the future and trial results would be published on a yearly basis. Standardized evaluation techniques used at multiple field test sites over multiple years would further enhance the selection of durable resistance which is often conditioned by environmental influences and exposure to diverse populations of Phytophthora infestans. In addition, adaptation of clones to various locations, agronomic characteristics and industry preferences could be assessed.

5. Empahsize breeding for late blight resistance as a priority over time even if "breakthrough" events in disease management occur in the interim.

Breeding new potato cultivars for resistance to late blight requires a long-term commitment of time and resources especially if durable resistance is sought. Although late blight evaluations were once part of breeding progams in the United States, this aspect of cultivar development was dropped in the 1950s with the advent of new chemical controls and the empahsis on processing and storage qualities. It is important that this history not be repeated. Funding commitment over a period of years is crucial.

Sophien Kamoun - 06:20am Feb 28, 1997
How many plants can P. infestans infect? There seems to be some confusion about this. P. Infestans is often presented as a specialized pathogen with a narrow host-range. However, there are numerous references suggesting infection of several plants including non-solanaceous ones (for example see the list in the "Phytophthora Diseases Worldwide" book).

Debbie Inglis - 10:56am Feb 28, 1997
Coordinating evaluation of potato cultivars and germplasm for resistance to P. infestans across many locations in Canada, Mexico and the United States needs structure. Ideas are needed on how this may best be accomplished.

Bill Brown - 12:09pm Feb 28, 1997
Debbie:
The way we operate the barely stripe rust international screening is through the National Small Grain lab in Aberdeen, ID. They collect all the material to be tested from the various public and private sector breeders. They then have a central record keeping system, arrange the record books and pack the seed. It then comes to me and we put it out to our cooperators in Mexico and Germany (prior years also included Ecuador and Bolivia). The key is to have one coordinating point/person and seed handling source (not necessarily the same for both). Of course you have to do all the phytosanitary papers, import permits, etc and it can get to be a real pain. Because of bulk, potatoes will be a logistical challenge--we could take 8,000 lines of barley on the plane with us to Bolivia! Multi-location, multi-country screening is critical to do but it is going to be a major challenge.

Howard Judelson - 06:24pm Mar 7, 1997
Sophien:
I think that the number of true hosts is overstated in the literature. Some lists of hosts probably include those that can be penetrated, as indicated by necrotic flecking, but not successfully colonized. But it is a mistake to consider only potato and tomato to be significant hosts. In addition to wild and uncommon Solanum spp., petunia is a good host of P. infestans. Since petunias are widely grown and distributed, that is something to think about!

Sophien Kamoun - 03:31am Mar 12, 1997
Howard:
What I find particularly surprising in the literature is the mention of non-solanaceous hosts for P. infestans. I think in many cases, sporulation was observed on such plants. In potato and tomato late blight field trials in Wageningen in 1972, an adjacent plot of Phacelia tanacetifolia (Hydrophyllaceae or water-leaf family) was infected by P. infestans and sporulation repeatedly observed on leaves and inflorescences (L.J. Turkensteen, Ph D thesis, Wageningen 1973). This sort of observations suggest that P. infestans may have a wider host-range than Solanum and related genera.

George Mackay - 11:13am Apr 10, 1997
Hallo ...I have only just found this site and it makes interesting reading. For those of you who are not already aware of it the International Potato Centre (CIP) Lima Peru has launched a Global Initiative on Late Blight..which seeks to address many of the questions that you appear to be interested in. Its initiative will include an international field trial for late blight and several organisations including at least two in North America are in the second year of a study designed to investigate GxE interactions on the expression of late blight resistance. For my sins I am at present chairman of the Steering Committee of the GILB...but if you want any more details and wish to be include on the GILB mailing list etc please contact Dr Ed French at CIP his E-mail address is E.French@cgnet.com

Regards George Mackay Head of Crop Genetics at the Scottish Crop Research Institute E-mail:
gmacka@scri.sari.ac.uk