The National Pesticide Info Center (http://npic.orst.edu/ingred/ptype/fungicide.html) says "Although they can slow or stop the development of new symptoms, many fungicides are designed only to prevent disease. These are not highly effective after symptoms have developed." That's my experience. Best to use them proactively rather than as a cure. Also, be aware that regular use of specific fungicides can result in development of resistant varieties. In that sense, blanket application is better than single-site application. Also, with regard to development of resistance, rotation among different fungicide families is advised. Daconil and Immunox are different chemical families. Note that Daconil isn't really systemic. It works by contact on leaf surfaces. So it'll eventually wash off. Immunox is systemic, and is actually absorbed in leaf tissue, and translocated within the plant. In that respect, there are guidelines on how many times you should use Immunox in any one season. That all being said, you might want to check that Immunox is approved for vegetables. As of a few years ago it wasn't, but that may have changed.

Fudge, lesson learned. It is going bad this year. Thanks!

Before you go ahead with a blanket application of chemical fungicides that you may not want to be consuming with your produce (I know I wouldn't !!), you might want to do a little research into powdery mildew, its causes and how to best control.

While there are many strains and forms of PM, some of which are very plant-specific, most have the same behavioral profile. Excess moisture is not a causal agent. In fact, it prevents germination of conidia/PM spores. One control method is to spray down the affected plants in midmorning to wash off and kill the spores. Midmorning or a midday timing is suggested to maximize foliar drying and reduce humidity.

High humidity can be a causal agent. It is therefore recommended to avoid overhead watering (especially very early in the day or late afternoon/early evening when humidity tends to be high) and focus instead on root zone irrigation. Maximizing sun exposure and ensuring good airflow will also help so do not crowd plantings and some thinning of foliage may be necessary to maintain good air circulation, which is really critical.

Most fungicides will only be prophylactic/preventative and not curative so any treatment should start before PM symptoms become apparent. Since PM is only a surface issue, several "natural" or organic approaches that change the pH of the leaf surface can be quite effective - baking soda, potassium bicarbonate and even milk have proved to be moderately to highly efficacious.

Various extension services publish detailed fact sheets on powdery mildew and its control that you might find helpful.....just Google.

Here's one to start: https://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html

In future years, plant PM-resistant varieties of vegetables where possible -- not a guarantee you won't get PM but it will lessen your odds.

"Any reason not to do this?" The best reason not to do this is IF you haven't read the entire instructions or don't completely understand them, and don't adhere to the limits . With this said most fungicides will state the maximum number of applications allowed, days between applications and total amounts allowed for the season. Please be aware of these restrictions, but I don't see an issue using Immunox, active ingredient Myclobutanil (3) and Daconil, active ingredient Chlorothalonil (M05) in an alternate scheduled .

I prefer to stay in the group of multi site activity mode of action (M) generally considered as a low risk group without any signs of resistance developing to the fungicides. I start with just copper (M01) if weather conditions warrant a proactive precaution, and if the disease makes it through copper I'll turn to Mancozeb (M03) and chlorothalonil (M05), and alternate without exceeding manufacturer instructions. These M code fungicides are nearly non toxic to mammals but must come in contact with the spores to be effective, and always a pain to get them on the underside of cucurbit leaves. I'm thinking I should get some Immunox for such occasions if I don't mind getting a little sick after spraying it and hated fish..