It is not a must have. A fungicide rarely is. Most All microbes make fungicides, bactericides and super complex chemicals that we'd wish we could copy to make a profit and cure disease. So having the right microbes and fungi around will provide your plants with all the antibiotics they need to ward off the bad guys. Then again, the bad guys do the same and it's a constant chemical arms race in the microbial world. Sometimes you need to use something from a bottle.
There are substitutes with the same active ingredient Metalaxyl but they - no matter the formula - have been deemed ineffective since the 1980's according to wikipedia (see the part above for part of the explanation).
Fungicides can break down and it doesn't make sense to have more around than you need.
If you're going to use fungicides, you do this in a certain fashion. Otherwise, you'd just be plain stupid. I'm not kidding, application of the wrong fungicide or the wrong technique of application or protecting yourself can damage you and your plants.
1. Identify the fungus - a rust is not a mildew, a mildew is rarely a problem. Make sure you know at least the family of the fungus. Always see if there's other ways of treating it. Sometimes a dash of vinegar, potassium carbonate, sulfur, limesulfur, copper sulfate or H2O2 is enough.
2. Look for fungicides that are known to kill this specific fungus. If there aren't.. Tough luck: Most fungicides will be pretty specific to certain fungi and will screw up your soil and foliar microbiome, so make sure they work. You can set a house on fire if there are ants in the garden, but that's not really helping. Same goes for biocides; if they don't work, just don't buy/apply it. Google scholar is your friend: fungus name / Bacteria name + antibiotic name.
3. When found, search the SDS or MSDS of the product. Look what it does to humans, to animals and to water and soil. Most of these things will be hazardous in the sense that they can sterilize a human, give you lung cancer faster than smoking six packs of cigarettes an hour, and make you blind. All those H- and P-sentences can be found on google.
4. Read the f-ing label. I can't stand the fact that we're all here on a forum reading hundreds of pages of bonsai information and freaking dare to ask "HoW dO I ApPly ThIS StuFF?!". It's on the box! It's on the label! It's in the manual! It's usually less than 40 sentences and there are images and stuff! Man, that really pisses me off. - I deleted a three paragraph rant here -
5. Apply it with caution and only when you need it. I know some people that treat extensively for needlecast on pines for instance and they still get 20 needles on a tree to develop it. I don't treat at all and I too get 20 needles per tree to develop it. They're spending time, money and wasting the (very local) environment to do absolutely nothing. I do nothing and.. Well, that's it.. Same result. It seems that either their timing is terrible, or that maybe, their fungicide isn't doing jack (except for breeding and genetically solidifying natural resistance).
