M Targerman, Y Mesika, U Adler, E Matan

Late  blight,  caused  by  Phytophthora  infestans,  is  one  of  the  most  devastating  diseases  of 
greenhouse grown tomatoes in Israel and elsewhere. For its suppression, growers frequently 
apply  fungicides,  but  it  is  not  uncommon  that  severe  epidemics develop  even  in fungicide-
treated crops. Determining the quantitative effects of the relevant factors on the pathogen may 
lead not only to better disease suppression, but also to reduction in fungicide use. The effects 
of various management actions (covering the soil with  plastic,  application  of  fungicides and 
sanitation), and their interactions, were studied in a series of experiments conducted in walk-
in  tunnels  and commercial-like  polyethylene greenhouses.  Under  conditions  of  the  western 
Negev  (south  west  Israel)  it  was  found  that foliar infection by the pathogen could be 
suppressed  by  covering  the  soil  with  reflective  polyethylene  (that  resulted  in  reduction  of 
relative  humidity  and  leaf  wetness  duration)  and  by  application  of  fungicides.  Under 
conditions  of  high  temperatures  (>20oC)  and  dry  foliage  the  infections  do  not  occur. 
Nevertheless, the pathogen progresses from infected leaf-blades via the petioles, to the stems, 
where it causes stem lesions. Stem lesions eventually lead to plant death. Observations made 
in  the  greenhouses  suggested  that  the  damage  resulting  from  stem  infection  is  more 
significant than that resulted from foliar infection. It is possible to prevent stem infections by 
sanitation, i.e. removal of  infected plant material. Moreover, it was observed  that  the rate of 
disease progression in infected leaves was reduced, and fewer plants died from stem lesions, 
when temperatures exceeded 30°C. Accordingly, avoidance from opening the side-opening of 
the  greenhouse  during  the  day  (which  resulted  in  increased  daily  temperature)  enabled  to 
further improve  disease  management.  In conclusion, the  combination  of  cultural means and 
spraying  control  agents  at  various  stages  during  the  growth  season  results  in  significant 
reduction of tomato late blight in greenhouses.