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1. Late blight occurrence and LB severity by major production area in the country. What crops are affected (tomato, potato, others)? Disease sources (seed, cull piles, volunteer plants, soil)?

In 2002, late blight occurrence across Canada was inconsistent with minor disease occurrence reported in processing and 'table' potato fields in only 5 of 10 provinces. Disease incidences were quite low at <1% of the farms and disease severity was also limited with levels ranging from trace to 3-5% foliar blight. No seed potato fields had late blight. 

Unlike 2001 when tomato late blight occurrence was quite high, late blight reports in 2002 were confined to potato crops.

In all cases, disease sources were most like due to infected cull piles or volunteer plants.  No infected seed or soil disease sources were reported.

In general, during 2002, late blight occurrence was nil to low across Canada and in locations where the disease occurred the number of infected fields and foliar disease incidences did not exceed low to moderate level.

2. LB impact (foliar damage, yield losses, tuber rot occurrences)?

Disease losses in the field were very limited with most of the disease in the field stopped due to the use of fungicides, disease plant removals and/or changes in weather to drier conditions.  As such, no yield losses were reported.  However, for a few farms, late season foliar (lower leaves and stems) disease occurrence did result in late blight tuber rot problems that were not observable until the crop was in storage for 2-4 weeks.

3. Fungicide use (amounts, types, etc.)?

Most farmers are using preventative fungicide programs (with contact fungicides, mancozeb, chlorothalonil, metiram, maneb) throughout the season beginning at 30 days post planting if disease risk is high (disease present in the area and favorable weather conditions) with a 5-7 day application schedule.  However, if disease risk is low (no disease present and unfavorable weather), fungicide use is initiated when the 'rows' begin to 'fill-in' with a 7-10 day application schedule.  Systemic or translaminar fungicides (propamocarb, dimethomorph, cymoxanil) are used in combination with contact fungicides if disease risk is high often in combination with a strong crop-scouting program. Metalaxyl is never used for late blight control but is applied in combination with contact fungicides, for tuber rot (pink rot and leak) control.  The increasing number of 'organic' farmers often use copper sulfate or Bordeaux mixture for late blight control. Unfortunately, where disease risk is high, these compounds are not stopping disease development.

4. Other LB control measures (resistant cultivars, forecast systems, none)?

While some 'export' seed and 'organic' farmers are using some of the newer late blight resistant, the majority of the seed, processing and 'table' potato farmers continue to grow the traditional, late blight susceptible cultivars (e.g. Russet Burbank, Shepody, Superior).  All major potato production areas across Canada utilize disease forecasting and rapid farmer notification systems to assess disease risk and provide recommendations for disease management.

5. LB control effectiveness (fungicide or host resistance failures)?

No new disease control failures were reported in 2002 in terms of resistant cultivars or fungicides.  However, both predominating pathogen genotypes in Canada are resistant to metalaxyl.

6. Pathogen strains (mating type, fungicide resistance, virulence factors, etc)?

Two major strains of the pathogen, an A2 mating type (US-8) and an A1 mating type (US-11) are present in Canada. These are both metalaxyl resistant to metalaxyl and tend to have an average of 5-7 virulence factors. Fortunately, in past few years, infected tissue samples have yielded only a single mating type although a few unique genotypes (both A1s and A2s) based on GPI, mating type, metalaxyl and virulence factor assessments, are found each year. As such, the pathogen populations across Canada appear to be are clonal (asexual reproduction predominates) in nature.  The significant increase in tomato late blight (US-6 genotype predominated) in 2001 was not observed during 2002.

7. Disease risks and/or major needs?

While two pathogen genotypes predominate in Canada, the infrequent occurrence of unique genotypes indicates that sexual reproducing population pockets or 'imported' sources of the pathogen continue to threaten.  Furthermore, late blight has also been reported on weed species (e.g. nightshade) in Canada and the United States of America for which many potato and tomato production areas are directly linked geographically and/or by crop marketing.  As such, the major needs include: monitoring of pathogen populations (genotyping, including virulence factors), disease source determination (particularly for non-traditional hosts), disease resistance source identification and utilization, and improved disease control for 'organic' farms.

8. Any publications on late blight in the country?

Peters R D, H. Forester, H.W. (Bud) Platt, R. Hall, and M.D. Coffey. 2001. Novel genotypes of Phytophthora infestans in Canada during 1994 and 1995. Am J Potato Research 78: 39-45.

Peters, R.D., H.W. (Bud) Platt, and R. Hall. 2001. Infectivity of asexual propagules of Phytophthora infestans in sterile peat exposed to various temperature regimes. Phytoprotection 81: 123-128.

Daayf, F, H.W. (Bud) Platt, G. Mahuku, and R.D. Peters. 2001. Relationships between pathotypes and RAPDs, GPI-allozyme patterns, mating types and resistance to metalaxyl of Phytophthora infestans in Canada in 1997. Amer J Potato Research 78: 129-139.

Platt, H. W. (Bud), F. Daayf, and A. MacPhail. 2001. Cross-Canada potato late blight survey in 2000. Canadian Plant Disease Survey. 82: 118-121.

Platt, H.W. (Bud) and G. MacKenzie. 2001. Response of potato breeding program lines and cultivars to late blight tuber rot caused by an A2 (US-8) mating type of Phytophthora infestans, 2000. American Phytopathology Society Biological & Cultural Tests for Control of Plant Diseases 16: 20-21.

Platt, H.W. (Bud) and G. MacKenzie. 2001. Response of potato breeding program lines and cultivars to late blight tuber rot caused by an A1 (US-11) mating type of Phytophthora infestans, 2000. American Phytopathology Society Biological & Cultural Tests for Control of Plant Diseases 16: 22-23.

Platt, H.W. (Bud) and Daayf, F. 2001. Canadian populations of Phytophthora infestans in 2000. Northeast Potato Technology Forum Proceedings. p 71-72.

Platt, H.W. (Bud) and Fouad Daayf. 2001. Potato late blight: Pathogen population dynamics across Canada in 2000. American Journal Potato Res 78: 476.

Platt, H.W. (Bud). 2002. Effectiveness of a natural plant product for potato disease control. Norquin (Can.), Saskatchewan.

Platt, H.W. (Bud). 2002. Reducing pesticide use with in-furrow application for disease control. Syngenta (Can.), Ontario

Platt, H.W. (Bud). 2002. Effectiveness of low environmental impact chemicals for disease tuber rot control. Dow AgroSciences (Can.), Ontario

Platt, H.W. (Bud) and I. MacDonald. 2002. Late blight tuber rot response of potato breeding program lines and cultivars to an A2 mating type (US-8) of the pathogen, 2001.  National Potato Breeding Program Reports.

Platt, H.W. (Bud) and I. MacDonald. 2002. Late blight tuber rot response of potato breeding program lines and cultivars to an A1 mating type (US-11) of the pathogen, 2001. National Potato Breeding Program Reports.

Platt, H. W. (Bud), H. Winston Johnston, Delmar Holmstrom, Ron DeHann, and Thomas L. Gallant. 2002. Residue management for potatoes under two-year rotation: ii. Effect on field and storage diseases.  Agri-Info. 02-9; Agdex 573.

9. Is tuber blight an increasing problem?

Tuber blight is a major problem when in combination with wet rot diseases such as leak, pink rot, bacterial soft rot, etc.  Tuber blight (a dry rot) remains a manageable disease in storage when these other wet rot problems are absent.  In terms of tuber blight incidence, greater levels appear to occur when wet weather is present during the final month of the growing season and during the harvest period if late blight is present in the area or in the lower leaves and stems.

10. Other comments

Recent findings that fungicides use for late blight control are linked to death of fish in nearby waterways following very 'heavy' rainfall events have demonstrated a very severe problem facing potato farmers (and in some cases, tomato farmers).  Both non-farm communities and government sectors are wanting to reduce the use of pesticides and for those battling late blight this poses a serious problem as alternatives for late blight control fungicides are limited in numbers and effectiveness. 

Prepared by H. W. (Bud) Platt, Agri-Food and Agriculture Canada. Email: platth(at)agr.gc.ca