Late blight prevention on tomato seedlings grown in greenhouse and high tunnel structures
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A. A. MacNab, Plant Pathology Dept., Penn State University

1.  Make sure late blight is not present on tomato, potato or petunia plants 
in any nearby greenhouse-like structures.  Late blight spores can spread
for miles by wind.

2.  People who have been near late blight infected plants should not enter 
greenhouse-like structures where tomato seedlings are being grown.  
Late blight spores can be introduced on workers hands, clothing, tools, etc.

3.  Do not grow tomato seedlings in structures where late blight has 
recently been present.  If late blight has been present in a structure, 
all plant material should be destroyed and surfaces sanitized at least 
2 weeks before seeding tomatoes.  

4.  Do not grow ornamental plants or potatoes in structures where tomato 
seedlings are grown.  Recent work has shown that the late blight fungus 
that affects tomatoes and potatoes, can also infect petunias and some 
other ornamental plants, and can also be introduced into greenhouses 
on petunia cuttings.  

5.  Minimize periods of time when environment favors late blight 
development within greenhouse-like structures.  It is important to focus 
on relative humidity.  Infection by the late blight fungus can occur when 
there are 10 consecutive hours with a RH of 90% or more; chance of 
infection increases with longer high-RH periods and higher temperatures 
(up to 80 degrees F).  Relative humidity should be kept below 90% as 
much as possible.  This is difficult, especially during long periods when 
rainy and cloudy conditions persist.  Usually ventilation and heating 
will lower relative humidity; unfortunately, this can be expensive when 
outside temperatures are low.  

6. A few preventive fungicides can be applied to tomato plants in 
greenhouse-like structures.  Included are sprays of maneb, mancozeb, and 
fixed coppers.  In addition, Previcur Flex, a material with some systemic 
properties, has a new label for use as a drench.  Some fungicides labeled 
for use in the field are not labeled for use in greenhouses.  Check the 
label to make sure use of the material in the greenhouse is not excluded; 
follow the label.  If the grower thinks there are no sources of late blight 
within 50 miles, one application of fungicide may be adequate before 
transplanting to the field.  However, if late blight is suspected to be 
present within 50 miles, multiple applications of fungicides could be 
useful.  

7. Tomato seedlings that are infected by the late blight fungus should never 
be transplanted, and should be destroyed.   

8. Any grower who suspects presence of late blight on tomatoes, potatoes, 
or petunias should contact the local extension office, the Penn State 
Plant Disease Clinic, or the PDA Plant Disease Clinic.  The diagnosis can 
be checked and additional assistance can be provided as needed.  

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A. A. MacNab,
Plant Pathology Department, The Pennsylvania State University.
Developed:  February 28, 2005

Where trade names are used no discrimination is intended, and no endorsement by the Cooperative Extension Service is implied.

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