Sydney NSW, Australia
Identification request
Questionable downy mildew diseases of squash-pumpkin crops - Tonga Islands
Here in the Tonga Islands (southwest Pacific):

Most field crops of our Buttercup-squash.pumpkin (C. maxima) and Butternut-squash (C. mochata) are heavily affected by this foliar diseases, which .. questionable?

Anybody outer there can help?

Squash-pumpkin productions for export this year 2018 really hit by dry-spell - drought (since late June, 2018). From June, we have not had any rain. It is forecast, that we in Tonga are facing another very prolong dry-spell for the rest of this year.

During my field visits and inpections ofthe said foliar disease, 
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.

Our day temperature is ranging between 20-25°C, and at night is around 14-19°C. Very dry and humidity is around 80%.

Farmers are using both protectant and curative (systemic) fungicides to control all diseases, presumably..... Fungicides used are:
  • 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.
It is unfortunate, that talking to farmers about seeds they planted, it is very uncertain from information that the variety has any resistance to the concerned disease...

Thank you all for your help.

Sione Foliaki
Vaini crop-research station, Sustainable Development Division
Telephone: +676-37474
Ministry of agriculture, Food, Forests and Fisheries.
PO box 14, Nukuʻalofa, 

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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 Fullerton
Principal Scientist
T: +64 9 925 7131
M: +64 212268131
F: +64 9 925 7001
The New Zealand Institute for Plant & Food Research Limited
Postal Address: Plant & Food Research
Private Bag 92169, Auckland Mail Centre, Auckland 1142, New Zealand
Physical Address: Plant & Food Research
120 Mt Albert Road, Sandringham, Auckland 1025, New Zealand

Posted on user's behalf

This does look like downy mildew.  More info here:

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 Lapaire Harmon, PhD
Extension Plant Pathologist, University of Florida -- 352-392-1795 – Bldg 1291, 2570 Hull Road, Gainesville FL 32611

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Dear Sione

I think Bob is right: it's not downy mildew, and for the reasons he's given" it's just too dry, and the spots are not angular enough, confined by the veins. It's not powdery mildew either!

Last year when I was there, your tomatoes at the research station had what looked like target spot caused by Corynespora. And right next to them were rows of staked cucumber collapsing from what I thought was the same cause. If I have to take a guess, I think you have the same thing on butternut squash.

Can you do a couple of things: i) takes some leaves at different stages of infection (importantly, some at early stages), and make herbarium samples of them so we can send them away for id; and ii) take lots more photos, and bring them to Fiji when you come on the 26 August.

If you have tomatoes and cucumbers with disease symptoms, collect leaves and press them too. Good if you can incubate in a plastic bag with a few drops of water to create a high humidity overnight.

If you need any information on making herbarium samples, let me know

All the best

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