2014-05-13: As an update to this article, readers are referred
to current advice from the Potato Growers of Alberta regarding destruction and disposal of infected plants.
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The
June 2011 issue of Gardens West, Prairie Edition had an interesting article written by Darlene Polachic about "
Late Blight" -- an article I am moved to bring to your attention and to expand upon.
Late Blight or
Phytophthora infestans is apparently a common fungal pathogen that was the root cause (no pun intended!) of the Potato Famine that took place in Ireland in the 1840s.
Late Blight attacks members of the
Solanaceae family which includes some of our favourite warm weather crops. In fact, the primary hosts of Late Blight are tomatoes and potatoes but it can also affect eggplants, peppers, petunias and various weeds such as nightshade species and wild tomato.
Late Blight is experienced worldwide and is often seen on both the West and East coasts of Canada where cool, damp conditions prevail. Polachic notes that, because of our typically hot, dry climate, it is less common to experience Late Blight on the Prairies. HOWEVER, the rainy, cool summer of 2010 provided the exact conditions for Late Blight to manifest: temperatures = 15-20 degrees Celsius and frequent showers and/or heavy dews that contributed to a relative humidity of 90% or more. (Polachic, 2011) While that was then and this is now, Polachic references experts that advise that the large quantities of snow this past winter suggest a strong probability of Late Blight occurring. Indeed, Alberta Agriculture & Rural Development (Spencer, 2011) has issued an
advisory to this effect noting that in 2010, Alberta suffered its first outbreak of Late Blight since 1993 and that it affected urban and market gardeners the worst.
Late Blight produces two spore types, sporangia and zoospores. Sporangia are formed on infected foliage and spread by wind and water. Zoospores form within a single sporangium and swim in films of water on the plant and in the soil to infect plant parts. The conditions described above can also be artificially created with the use of overhead and sprinkler irrigation. Under ideal conditions, spore formation can occur within 10 hours. Foliar materials (i.e., leaf material) is not required to foster spore production -- spore production can take place on whole tubers or cut seed potatoes and by dispersed by handling. Spores can spread in storage as well as in cull piles, undecomposed compost heaps, and in seed potato pieces. (Alberta Agriculture & Rural Development, 2011)
What do you look for? The first sign of trouble will be a dark, water-soaked area usually occurring at the margin of the leaves and moving in towards the centre. Lesions are not contained by leaf veins (unlike Early Blight) and young lesions may have a yellow edge or margin. Lesions usually turn brown or black in colour and may become brittle when they dry out. (Alberta Agriculture & Rural Development, 2011).
Polachic (2011) advises that you look for "white cottony growth on the underside of infected leaves." This is the spore formation. The disease will then advance down the stem turning the stem and
petioles dark brown or black causing the plant to droop and the leaf canopy to have a "flagging" look.
Alberta Agriculture & Rural Development (2011) also notes that reddish-brown, irregular, sunken lesions may develop on the tomato fruit (usually on the shoulder of the fruit) or potato tuber (usually around the eyes) which Polachic (2011) describes as "slimy." She goes on to say that a potato may appear normal but once cut into, the flesh displays the characteristic reddish-brown rot. With tomatoes, however, the Blight displays on the shoulders of the fruit as reddish-brown lesions surrounded by yellowish halos. As the weather drys, the lesions will stop growing and turn brown and brittle.
(NOTE:
here are some images of Late Blight infested potatoes and tomatoes.
Alberta Agriculture & Rural Development also have some helpful images on their website.)
What do you do if you have Late Blight? Both Polachic and Alberta Agriculture & Rural Development advise that suspect plants should be pulled out immediately, bagged, removed from the garden and destroyed. While I might suggest you should NOT COMPOST diseased materials, Alberta Agriculture & Rural Development (2011) suggests composting is an option IF you can quarantine the infected material (either bagging it or covering it with a tarp) until it is frozen and the plant material has broken down fully.
Nearby plants, which may also be infected should be removed or, perhaps less drastically, monitored for signs of the disease. (Polachic, 2011)
Are there preventative measures to take? Polachic (2011) notes that the spores of Late Blight die in three hours of low humidity. Therefore, if the weather turns dry, the disease may be checked. However, she also provides the following advice for gardeners:
For potatos:
- water your potato plants at the soil level as overhead watering will wash the spores into the soil where they will infect the tubers.
- water early in the day to allow the foliage to dry before evening.
- hill your potatoes to protect the tubers from infection -- tubers situated close to the soil surface are likely to be infected.
- discard any tubers that show greening.
- if you are not sure if your potatoes have been infected, then leave your potatoes in the ground for a week. If the infection is present, they will have rotted -- and then you will know for sure!
- Do NOT use potato tubers from infected plants as seed potatoes the following year -- use new seed potatoes. Late Blight can winter over on infected tubers and spread during storage.
For tomatoes:
- keep your tomatoes pruned and staked to allow airflow through the leaf canopy -- excessive vine growth and over-fertilizing invites Late Blight.
- if the fruit is red without blemishes, then the fruit is good to eat. Polachic notes that you can dip your fruit in a 10% bleach solution to kill surface spores but this solution will not work if the spores have already germinated.
- if the fruit is green and without blemishes, do not attempt to ripen them as they will simply rot. Polachic advises using them up immediately in your favourite green tomato recipe. (NOTE: I have eaten cooked green tomatoes and they are quite delicious.)
- Destroy (DO NOT COMPOST) the plant.
- Late Blight is not carried on tomato seeds (for the seed savers in the crowd) but it can be introduced on transplants.
Are there cultivars resistant to Late Blight? Apparently not although Polachic (2011) does say that Russet cultivars (of potatoes) are more resistant than red cultivars. In the case of tomatoes, there are cultivars that are advertised as resistant but these cultivars have not been tested on the Prairies.
What else can you do? There is a apparently a gardening product called
Serenade Garden that helps. The active ingredient is a bacteria called
Bacillus subtilis which feeds on fungi. The advice around its use is to spray early and about every 10 days and ONLY AFTER the plant has reached the two-leaf stage (as anything younger will not survive). Another option is treating with copper spray which must be repeated every time it rains or the plants are watered.
As a side bar, in her article, Polachic points out some subtle differences between Late Blight and another gardener's nemisis, Early Blight:
- Early Blight presents as lesions with concentric rings within the border of the lesion.
- the progression of Early Blight is blocked by the veins on the leaf.
- Early Blight can be on the plant for several weeks without causing damage.
SOURCES:
Alberta Agriculture and Rural Development (2011 April) Late Blight of Potatoes and Tomatoes. Retrieved 2011 June 9 from
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/faq13559.
Polachic, D. (2011 June). "Will Late Blight strike again?" Gardens West (Prairie Edition), 25 (5), 64-66.
Spencer, R.C.J. (2011, April 8). AB Late Blight Awareness Initiative. Retrieved from
http://organicalberta.org/organic-alberta-news/ab-late-blight-awareness-initiative.
