Late Blight Epidemiology Discussion Groups

Potato Late Blight On-line Workshop
The Fungus and the Disease
Discussion Forums

Epidemiology

At the Tucson workshop, about 80 people attended the epidemiology breakout session, making it one of the larger groups to discuss late blight research needs. Following the lead provided by Dr. Bill Fry's presentation, the epidemiology discussion group outlined the discussion based on the following topics: a) between-season biology, b) establishment of initial infections in field/production areas, c) development of field epidemics, d) spore dispersal, and e) infection of and disease development in potato tubers.

In the two hour discussion, many ideas were put forward. Of course, the liveliest discussion centered on topics for which the least is known. After consolidation of recorded ideas, the following priorities were developed by the group:

* Assess the relative importance of inoculum sources that initiate epidemics; e.g., cull piles, seed tubers, volunteers, home gardens, and transplants. In addition, the significance of oospores in the disease epidemiology needs to be determined.

*Investigate the epidemiology of tuber blight. This includes pre- and post-harvest development of tuber infections, the effect of foliar disease severity on tuber rot epidemics, and the development of standard protocols for tuber rot assessment.

*Determine the importance of lesion type on the initiation and continuation of foliar epidemics. This includes potential genotype differences in the kinds of lesions formed (e.g., stem versus leaf), the importance of latent infections, and how lesion type interacts with microclimate in the initiation of field epidemics.

* Investigate long distance dispersal of sporangia. In particular, how it influences regional epidemics.

The research needs developed by the epidemiology group overlapped with several other discussion groups. The need to investigate the relative importance of inoculum sources was similar to recommendations made by the biology, cultural control, and tomato late blight discussion groups. Similarly, the need to study tuber blight was also given high priority by the biology and cultural control groups. Recommendations to investigate spore dispersal and the importance of lesion type were related to research needs highlighted by the disease forecasting discussion group.

Steve Wagner - 06:04am Feb 19, 1997
What is known about the development of late blight from infected seed pieces after planting? Does the disease progress in a predictable fashion?

Hamm Phil - 09:13pm Feb 25, 1997
In response to Steve's question- There are a number of studies, all probably using the old US1 strain that suggest seed infection resulting in above ground infection in be a rare event. However with this disease a rare event would be enough to serve as primary inoculum and cause an epidemic.

With new genotypes, work by Dr. Walt Stevenson from U of Wisconsin, shows at least one (US8) may be more able to cause above ground infections because of how they infect tubers. I do not believe this data has been published.

A late blight epidemic progresses based on the availability of a susceptable host, the pathogen being present, and a favorable environment. The later is very important. If wet and mild, the epidemic will move at a very high rate.

Bill Brown - 03:25pm Mar 2, 1997
Phil--while we all have traditionally accepted the environmental limiatations as being cool and wet, are we sure with the new mateing types? Where it has been found on the eastern Colorado plains it is neither cool or wet. Granted we get moisture under center pivot, but leaf surfaces dry very rapidly (yes there is sometimes heavy dew loads down in the canopy) and temperatures are up in the high 80 and 90s. Do the new mateing types have higher temperature thresholds? Has anyone documented this?

Philip Hamm - 10:12am Mar 4, 1997
Bill:
Notice that I said "wet and mild" not "wet and cool" conditions favor late blight epidemics. I have seen active infections with the temperatures have been as high as 110 F for a week. The fungus isn't active mid day but in the morning hours the beast seems happy enough. There has been much speculation as to the cardnal temperatures of these new genotypes, particularly in reference to infection and sporulation. I am unaware of any published information that clearly shows these new genotype more "adaptive" related to temperature.

There is evidence that two of these new genotypes, US8 and US11, are more aggressive (Miller et al, WSU). One of the characteristics we quantified was the genotypes ability to produce spores in higher numbers. Dennis Johnson has found environmental conditions in center pivot irrigation systems in the Columbia Basin to be much of the time supportive of late blight developmemt. The critical factor is the conditions outside the circle.

Dave Lambert - 08:05am Mar 10, 1997
New strains appear to develop more rapidly at low temperatures than old ones. Walker and Cooke (1990) reported faster growth rates at 5°C for European isolates described as phenylamide-resistant than for sensitive ones. Willie Kirk will be publishing on the progress of tuber infection at low (storage) temperatures. He is finding that our new isolates also are relatively more active in tubers at lower temperatures than are the old US-1 strains. (This is in addition to the general rapid development of new clones at moderate temperatures.) We have found that thermal death points for agar cultures in a high-temperature incubator differ by less than 1°C among US-1,-6,-7 and -8.