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Leaf spot caused by Phytophthora nicotianae

Leaf spots and stem symptoms caused by Phytophthora nicotianae very closely resemble those produced by the related pathogen Phytophthora infestans, which causes late blight.  These pathogens differ substantially in the quantity of spores (sporangia) they produce, which at least partly accounts for why only late blight is a highly contagious, destructive disease.  Sporangia were very hard to find on the leaves photographed here, even following 1-day incubation in high humidity (inside plastic bag on wet paper towel).  P. nicotianae can cause pink rot in tubers.  These leaves were found in crops on Long Island during July in 2013 and 2015.  Symptoms have also been reported in other states recently in the region.

Leaflets below photographed on 21 July 2015.

Phytophthora nicotianae

Phytophthora nicotianae

Below are the same leaflets after they were in a plastic bag for only 1 day.

Phytophthora nicotianae

Phytophthora nicotianae

Leaflets and stem below photographed on 30 July 2013.

Phytophthora nicotianae

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