Fungicides

Personally I prefer not to use poisons at all but sometimes when you want to save your plants and you can’t get effective results with more friendly potions you’ll need to bring out the heavy artilliary. It is very important not to overuse fungicide. Apart from potential risks to your own health, we really don’t want the nasties to build up resistance to the cure.

Some simple disinfectants such as phosphoric acid applied weekly during at risk times can prevent infection.
Over time I will add some links to information about various chemicals that can be applied. Always follow the instructions for these products very closely. It is never a case of more is better and nor to save a bit by cutting the quantity short. The effectiveness will be less and the spray will have little result.

https://www.yates.co.nz/products/disease-control/concentrates/yates-liquid-copper-fungicide
https://www.yates.co.nz/products/disease-control/concentrates/yates-copper-oxychloride/
https://www.yates.com.au/products/disease-control/concentrates/yates-anti-rot/
https://pnwhandbooks.org/plantdisease/pesticide-articles/fungicidal-activity-nutritional-value-phosphorous-acid
“Control of Oomycetes
It is well documented that phosphorous acid is able to control diseases caused by organisms that belong to the Oomycota (or oomycetes) that are on agronomical crops. Oomycetes (a group of pathogens that include water molds and downy mildew) are actually not fungi but are frequently grouped with fungi, because they form structures (filaments) similar to the ones that fungi make. In reality, oomycetes are fungal-like organisms that differ from fungi in that their cell walls do not contain chitin but a mixture of cellulosic compounds and glycan. Another difference is the nuclei in the cells that form the filaments; each have two sets of genetic information in oomycetes (diploid) instead of just one set as in fungi (haploid).
For most practical purposes, the oomycetes are grouped with fungi. Compounds that control plant pathogens belonging to the oomycetes are often called fungicides. It is important to distinguish between fungi and oomycetes; chemicals that are used to control one will often not be effective against the other, depending on their different biology. Several important plant pathogens belong to the oomycetes; the one with the most economic impact is Phytophthora infestans, which causes late blight of potato.
Phosphorous acid has both a direct and an indirect effect on oomycetes. It inhibits a particular process (oxidative phosphorylation) in the metabolism of oomycetes. For instance, phosphonate compounds are ineffective against phosphonate-resistant oomycetes. In addition, some evidence suggests that phosphorous acid has an indirect effect by stimulating the plant’s natural defense response against pathogen attack.
Efficacy
A major factor in the ability of phosphorous acid to control oomycetes for long periods of time appears to be its chemical stability in the plant. Phosphorous acid does not convert into phosphate and is not easily metabolized. The stability of the different phosphonate-related compounds may depend on environmental factors such as climate or crop type. Because phosphonate is systemic and stable in the plant, it should be applied infrequently. Plant species may differ in uptake and translocation of phosphonate, and there is great variation in sensitivity of individual P. infestans isolates to phosphonate compounds, which may negatively impact the effectiveness of phosphonate.”