Growing onions for success in preventing Downey mildew with Phoscare and Mycorrhizal fungi.

In this blog, we’re looking at the unique relationship that allium crops have with Mycorrhizal Fungi and how Phoscare, a phosphite product, offers growers tools to prevent the impact of diseases like Peronospora sparsa (otherwise known as Downey mildew).

Allium crops are one of New Zealand’s largest exports, with 85% of our annual harvest going to markets like the Netherlands.  Growing onions of high quality that keep well and are nutritionally dense is of vital importance to the 85 dedicated growers in Aotearoa, New Zealand, and Mycorrhizal fungi, along with Phoscare, a phosphite product, should certainly be considered early at planting, to achieve that.

There are around 5,000 hectares dedicated to growing onions each year, with most of the crop coming from the clay loam soils of Pukekohe, but you’ll find onions being grown in Canterbury, Hawkes Bay, the Bay of Plenty and Waikato.  Growing onions in these regions face similar issues in terms of disease pressure like Downey mildew, nutritional requirements and of course, changes in weather patterns.  

Planting with Mycorrhizal fungi.

Because they are shallow-rooted, growing onions or allium crops in general are highly mycorrhiza-dependent.  These beneficial organisms facilitate soil nutrient and water uptake by plants and can improve tolerance to some abiotic stresses such as frost, drought and salinity, as well as biotic stresses pertaining to certain soil-borne pathogens. 

The addition of Mycorrhizal fungi, named Rootella, during seeding of a new crop will increase strike rate, reduce plant loss, and mycorrhizae act as a physical barrier to detrimental pathogens. Mycorrhizal fungi give plants better access to nutrients locked in the soil, especially Phosphorus, which is so important to onions to ensure consistent growth and timely maturity.  Providing an array of beneficial organisms that utilise different mechanisms against disease is an important factor in prevention and one of the reasons why pathogens do not become resistant to these natural solutions.  

Nutritional requirements and disease prevention.

The nutritional requirements of onions and other allium crops include, of course, the three macronutrients required for all plants, Nitrogen, Phosphorus and Potassium (N-P-K).  However, the balance of these nutrients and the timing of application is crucial to both crop and environmental health. 

Too much Nitrogen can delay maturity and is believed to affect storage life and encourage pests and disease.  Excess N also means the plant sends all its energy to growing tops rather than bulbing, not to mention the problems associated with Nitrogen leaching.

Phosphorus is one of the most difficult nutrients to supply as it is often ‘fixed’ and unavailable to plants, with approximately only 20 percent utilised in the year of application, it can be wasteful and expensive.  While Mycorrhizal Fungi can greatly assist with foraging of Phosphorus, applications of a foliar P and K fertilizer such as Phoscare will supply a highly systemic form of Phosphorus and Potassium in the phosphite formulation, quickly recognised and utilised by the plant.

Phoscare is produced using non-traditional technologies. Phosphorous acid, instead of phosphoric acid, is used as the phosphorus source. The product derived from using phosphorous acid is phosphite instead of phosphate. The key here is that phosphite uses a three-pronged approach against fungus-like pathogens.  

“Firstly, phosphite targets the pathogen directly and inhibits its growth. Secondly, it can act through the plant and increase its natural defence against dieback, and thirdly, it can cause a ‘greening effect’ on the plant by amping up the production of proteins that are associated with photosynthesis” (Andronis. 2024)

Sprayed onto the soil and plant, Phoscare will move rapidly via both the xylem and phloem, i.e. systemically throughout the entire plant. Many pathogens thrive in a high nitrogen environment, but Phoscare has zero nitrogen and high P & K.  Also, Phoscare works by activating the phytoalexins within a plant, ramping up its own defence system. Having a highly stable product that does not degrade in the drum is an important and overlooked aspect in choosing a quality product.

It’s important that trace elements are not forgotten in the haste to cover NPK requirements, and while needed in smaller quantities, they also play a critical part in producing a healthy quality yield. Studies into micronutrient removal rates by onions show that boron and iron are particularly important. Added to this, Magnesium and Zinc have also been found to positively affect the size and quality of onion crops. Sulphur to increase the bulb yield and dry matter but also improve onion quality, especially pungency and flavours. A holistic approach that covers all mineral requirements as well as an array of beneficial organisms applied at planting and especially Mycorrhizal fungi will see healthy, clean allium crops. 

Putting it all together.

More information about the products we have talked about in this blog are available in the links below and if you have any questions, simply email us at

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