Small Source Leads to Major Outbreak: Late Blight on Long Island in 2011

The outbreak of late blight occurring on Long Island, NY, during summer 2011 was arguably the worst ever in that area, even worse than in 2009.  A major lesson learned from the event was the large impact that a small source can have, an important fact that needs to be understood by all farmers and gardeners growing tomatoes or potatoes.  Late blight is a community disease that everyone has to manage.

Identification of the responsible genotype through the USABlight project provided valuable information for determining the source.  Understanding the pathogen source for outbreaks is useful for improving management.  All 15 samples of leaves collected from several locations on LI were determined to be the same genotype (US-23).  It’s possible that there was a single source of this strain.  This genotype had not previously been detected on LI, therefore the source was probably not infested potato tubers surviving on LI from the previous year.  US-23 had not been detected in upstate NY or ME before 2011.  These locations are the major sources of seed potatoes for LI potato farmers.  The two most probable avenues for entry of US-23 onto LI are infected potato tubers or infected tomato transplants purchased by gardener(s). The pathogen attacks both plants.

Late blight also developed first in gardens on LI in 2009 and 2010. Tomato transplants were the source in 2009, which lead to a major epidemic. Potatoes planted in a garden were identified as the most likely source for the first occurrence of late blight in 2010. Fortunately, the gardeners in 2010 recognized the symptoms and promptly submitted samples for confirmation to the local extension office and then destroyed all of their tomatoes and potatoes; there were no additional occurrences of late blight in the area.

The 2011 epidemic on LI probably started during late May to early June in at least one garden in Sagaponack.  One grower in this town observed severe symptoms in his tomatoes on 22 June. There were numerous rain events during June 2011, plus long foggy periods are common on the South Fork of LI where late blight started to develop.  The grower had not inspected his first tomato plantings during most of June as outbreaks of any disease are very unusual that early in the growing season, and he was focused on trying to keep up with his other planting activities during the few days that it was not raining.  This farm could not have been the start of the epidemic because the farmer does not grow potatoes, he grows his own tomato plants from seed, and the pathogen cannot survive on tomato seed without there being two genotypes together of opposite mating type to produce a special spore type. Fairly new symptoms were found on tomato and potato at several farms on 27 June near the severely affected plantings.  On 7 July symptoms were found about 25 miles away on the North Fork of LI.  The pathogen spread throughout most of the agricultural region on LI and on to Rhode Island by early August.  Late blight continued to be found at new farm and garden locations throughout the growing season, including during a period in mid July with unusually high temperatures.

Identification of the responsible genotype through the USAblight project also provided valuable information for management.  US-23 is sensitive to the fungicide mefenoxam.  Since the 1980s, the pathogen population in the USA has been dominated by strains resistant to this fungicide, rendering it ineffective.  Several other mobile, targeted fungicides have been registered recently.  However, none of them are similar to mefenoxam in terms of mobility in plants or efficacy.  With knowledge of the strain causing late blight in 2011, growers included mefenoxam in their fungicide program for the first time in several years and were able to manage late blight more effectively.

Submitted by Meg McGrath, Cornell University, February 2012