Questionable downy mildew diseases of squash-pumpkin crops - Tonga Islands
I could not tell if the yellowing spots are round or angular on the top surface of the leaf. Spots are rapidly turning brown, but often limited by the leaf veins. I have not done any Incubation of the leaves at 20-25°C and high humidity overnight to promote development of the spores.
- Protectant products: coppers, e.g., copper oxychloride; mancozeb; chlorothalonil. Active Ingredient: 750g/kg mancozeb in the form of a water dispersible granule;
- Systemic products: metalaxyl; phosphorus acid. i.e. 640g/kg Mancozeb + and 80g/kg Metalaxy.
- BENOMYL 50% WP Active Igredient: 500g/kg benomyl in the form of a wettable powder.
Responses
Hi Sione
Nice to know you are still active.It is not easy to tell what it is but am not convinced it is downy mildew. The lesions are a bit too rounded and many tend to have yellow halos. Some are also associated with main veins. On the younger leaves the lesions are starting off as small spots.
With downy mildew the lesions are quite large and very angular at the time they first appear, first as lighter green angular spots which then turn yellow. They do not have a halo. Downy mildew sporulates on the lower side of the lesion in the very early stages of spot development, before the tissue turns yellow. You will see dark purple or black masses of spores on underside of the early stage spots. It grows only on living tissue and will not sporulate on the dead tissue if you incubate it in a humid container. Nor will you see spore masses on dried out spots on the leaves.If you are having very dry weather it is also unlikely to be downy mildew which requires wet weather to infect. However I note the night time temperatures are quite low and if there are heavy dews then it could promote mildew.Without being able to examine it closely I suspect it might be something like Pseudocercospora.
In any event the fungicides you are using, apart from phosphorus acid and metalaxyl which are specific to oomycetes, should control both downy mildew and most fungal diseases. With protectants you will need to ensure good coverage of both sides of the leaves, very difficult with cucurbits. Benomyl has strong curative action provided the fungi are not resistant to it already. Maybe you should look at mixtures of triazoles and strobilurins as alternative curatives. I note that the fruit are well advanced so you may need to consider the withholding periods of the fungicides you are using if they are near to harvest.
Kind regards
Bob FullertonPrincipal Scientist
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This does look like downy mildew. More info here: http://www.omafra.gov.on.ca/english/crops/facts/10-065.htm.
This one is tough to manage once the disease gets going, even with proper application. The mancozeb and chlorothalonil should work, but these have to be on before the plant is exposed to the pathogen. Something I’m sure you appreciate, but is hard to get farmers to do when money is tight – “why spray when you don’t see disease?”.
Best regards,
Carrie
Extension Plant Pathologist, University of Florida
clharmon@ufl.edu -- 352-392-1795 – Bldg 1291, 2570 Hull Road, Gainesville FL 32611
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