Greenhouse Canada

Late blight threat and greenhouse crops

May 2, 2014  By Alberta Agriculture and Rural Development

May 2, 2014 — Over the last few years, there has been a great deal of
concern in Alberta surrounding a serious disease called Late blight that
affects mainly potatoes and tomatoes.

May 2, 2014 — Over the last few years, there has been a great deal of concern in Alberta surrounding a serious disease called Late blight that affects mainly potatoes and tomatoes.

This disease is caused by a fungus-like microorganism called Phytophthora infestans.


The excellent conditions for disease development, combined with the presence of the pathogen, has resulted in continuing outbreaks of Late blight in commercial, market garden and urban potato and tomato crops throughout parts of Alberta.

A number of different strains of the pathogen have been identified in different years, each being more or less aggressive on either potatoes or tomatoes.

Recently a specific strain (US 23) of Late blight, which is particularly virulent on tomatoes, has become more prevalent, increasing the risk for greenhouse tomato growers.

In 2013, Late blight advanced across southern Alberta, coming close to greenhouse tomato areas.

For 2014, this disease continues to be a risk for all Solanaceous crops (potato/tomato family) grown in Alberta.


While most of the attention and focus is placed on field-grown crops, due to the size and scale of the industries that may be affected (e.g. commercial potato, market gardens, etc.), greenhouse producers of tomatoes or tomato transplants, as well as eggplants and petunias, should be concerned about their ability to potentially impact other industries or be adversely affected by Late blight.

Producers should monitor for Late blight in their crops, from the perspective of buyers, sellers and producers of plants.

• As buyers, producers are at risk of receiving infected plants from other regions, which may significantly affect their own production.

• As buyer/sellers, receiving infected plants creates the possibility of introducing diseased plants into areas where it could easily spread to other crop industries, which can start the disease cycle locally.

• As producers, if the disease is introduced in the province, there is the potential of having their crops infected as the season progresses, either affecting production or furthering the spread into other crops.


While potatoes and tomatoes are generally considered the primary crops that are affected by Late blight, crops like petunias, peppers and eggplants may also be infected and/or can spread disease to other, more common host crops (e.g. tomatoes), particularly if they are housed in the same greenhouse.

Spread between multiple host crops can certainly occur in greenhouse situations.

It has been suggested that crops such as petunia are not likely to be entirely wiped out by Late blight, unless they are young seedlings (highly sensitive).

However, older plants can serve as inoculum for the spread of disease within and out of a greenhouse environment.


Scouting / monitoring can be done at the same time as plants are monitored for insect pests and other diseases.

Initial symptoms of Late blight are typically noted on older leaves, appearing as dark, water-soaked lesions, sometimes with yellow edges, that move in from leaf tips/margins, becoming brown and brittle within a couple days.

Late blight lesions are not contained by the leaf veins.

In crops such as petunias, lesions may not develop as rapidly and may resemble other foliar leaf diseases, depending on the stage of the crop at infection and the level of infection.

In high moisture/humidity situations, a small amount of sporulation (observed as white, fluffy growth on the edges of lesions) may be visible on the underside of affected leaves.

Other diseases will likely form spores much more rapidly than the Late blight pathogen (e.g., Botrytis cinerea, the gray mold pathogen).

Late blight develops most quickly in wet/humid conditions and can spread very rapidly through tomato plantings or very young petunia seedlings. Plants may be rapidly defoliated and die.

Specific strains of Phytophthora are more aggressive on tomatoes (US 23), and will often attack the fruit readily; therefore, producers should watch plants for both foliar and fruit symptoms.

Infected fruit may have irregular, sunken lesions. Tomato fruit rot can penetrate into skin of the fruits, causing rot and discolouration of the internal tissues. The rot often has a reddish-brown colour.


Careful monitoring of incoming, growing and outgoing plant material is one of the best strategies for managing Late blight within a greenhouse.

Producers should consider separating different host plants as much as possible, particularly if there is a risk of disease on one of the crops.

Consider culling poorer quality plants or carefully screening for potentially infected material. Dispose of diseased material by burial, burning or freezing. Dying plant material can still transfer spores to living plants, continuing the disease cycle.

The Late blight pathogen thrives in warm, wet and/or high humidity conditions; therefore, careful ventilation can help to keep humidity at reasonable levels and can prevent condensation and prolonged periods of leaf wetness.
Overhead watering will increase disease spread; this should be addressed if there is a risk that disease is present.

Protective applications of registered fungicides are appropriate in high risk situations; however, applications are not curative.

Late blight is a community disease. It will require effort on the part of all industries to return Alberta to a Late blight-free status. If you want to know more about Late blight or have questions or concerns, please call 310-FARM (3276) for assistance.

For more information on Late blight identification and management, see the Frequently Asked Questions document – Late blight in Potatoes and Tomatoes.

Reprinted with permission.

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