Make more of micronutrients – Fruit & Vine

They may be needed in far smaller quantities than macronutrients, but micronutrients like zinc, iron and manganese are no less important given their pivotal role in many metabolic processes, including photosynthesis, chlorophyll production, enzyme activity and cell development.

Requirements vary throughout the season, so it is essential growers are aware of what vines need at key times and understand the requirements of individual sites to avoid deficiencies occurring. The aim is to be proactive with all nutrition, rather than reacting to deficiency symptoms, as by that time, damage may already be done, explains Kent- based Hutchinsons agronomist, Rob Saunders.

One of the most important micronutrients in vines is boron. It is key to plant reproduction, and alongside manganese and zinc, has also been shown to help improve sugar concentrations in grapes. Boron and calcium, have key roles in protecting vines against botrytis infection too.

“Boron plays a central role in pollen tube development and pollen viability, so any deficiency could compromise fertilisation and fruit set,” says Mr Saunders. “This is often characterised by uneven grape development, known as millerandage (other causes include cold, wet weather leading to extended flowering).”

Other symptoms can include chlorosis (yellowing) around the outside of the leaf, which may turn red-brown colour before leaves die off.

Boron is often not held well in certain soil types (particularly sandy soils), so he advises growers to monitor vines and ensure levels are sufficient before flowering starts. Deficiency risk is also higher in highly acidic or alkaline soils.

Where millerandage – also known as ‘hen and chick’- has been found in the past, take note of affected areas, and where necessary, apply boron from bud burst to rectify any deficiencies before flowering commences. It can also be applied to green leaves post- harvest to build levels for next season if required, he notes.

Zinc has a significant role in many physiological functions, including helping with development of healthy green foliage, improving fruit set, and in the formation of auxins, which help stem elongation and growth regulation.

Vines grown on high-pH soils containing low organic matter and excess phosphorus can be more prone to zinc deficiency, while waterlogging can also restrict root growth and availability to the plant- although this applies to many other nutrients too, says Mr Saunders.

Deficiency symptoms typically include light green chlorosis on the interveinal areas of the leaf, smaller than normal leaf blades, and small grape bunches with only a few set berries. A lack of zinc has also been implicated in millerandage as well. Research shows that foliar nutrition products such as Zynergy (containing copper, zinc and sulphur) and phosphites, such as Phorce, can boost the vine’s natural defence mechanism, markedly reducing downy mildew risk.

Magnesium is another micronutrient needed for chlorophyll production, but many rootstocks (notably SO4) can struggle to readily absorb magnesium from the soil, so deficiencies can often occur, says Mr Saunders.

Foliar magnesium applied as soon as enough leaf area has emerged is therefore a routine requirement in many situations. “There are two main treatment options, the most popular being magnesium sulphate, which is low cost and proven to work well. Another option, where growers require additional nitrogen, as well as magnesium, is magnesium nitrate, which offers a more plant-available nutrient form than sulphate.”

Iron is also used for chlorophyll production and leaf development, but can help berry development and lead to increased sugar levels in grapes too.

It is less available in calcareous soils, as calcium carbonate locks-up iron, causing “lime-induced chlorosis”. Rootstocks suited to these soil types should be used to mitigate this risk, Mr Saunders advises.

Iron deficiency symptoms include yellow interveinal chlorosis, often visible on younger leaves first, with the most severely faded around the edges. Very severe deficiency will also show up on older leaves and shoots.

Where iron is required, Mr Saunders says EDTA chelate forms are cheapest, but they are generally not as plant-available or effective as iron metalosate, which supplies amino acid chelated minerals in a ‘softer’ form that can be used immediately by plants.

Manganese plays a positive role in photosynthesis and enzyme activity, so contributes to a healthy green canopy, which in turn leads to better berry development and improved yield.

Manganese has become something to watch for in recent years following the loss of mancozeb. The active contained manganese, so when it was applied for early Phomopsis control, it used to also contribute some manganese to the leaf.

Since mancozeb’s ban, Terramap soil analysis combined with tissue testing is highlighting more situations where manganese levels are sub- optimal, says Mr Saunders.

Deficiency symptoms include yellow interveinal chlorosis, while the fine veins stay deep green. Leaves will also be smaller in size. As with some other nutrients, sandy, organic, and very alkaline soils can be more prone to manganese issues.

Foliar manganese sulfate can give immediate effects. Applying treatments directly to soil is not recommended, especially in high pH, or high organic matter soils.

Molybdenum is used in floral development and some research shows it also has a role in the mobilisation and accumulation of sugars in grapes.

“There is a range of nutritional products available for use throughout the season, so talk to your agronomist about the most appropriate options.”

Copper is needed in small amounts for nutrition, as it has a role in physiological processes, such as photosynthesis and respiration. However, excessive copper can be toxic to plants and most nutritional requirements will be met by the use of copper-based products for disease control.

Regular soil testing provides a useful indicator of overall nutrient status, but Mr Saunders say this should be supported with petiole analysis at early flowering to build a more complete picture of macro and micro nutrition, and crucially, highlight any potential deficiencies in time to respond.

“When petiole sampling at flowering, remember to take the petiole from the leaf opposite the lowest bunch position on a shoot, selecting representative vines of the same variety. If petiole analysis reveals an urgent need for certain nutrients, foliar nutrition in metalosate form has been shown to be a quick way to get nutrients into plants. Mineral nutrients are chelated with amino acids, which protects them from adverse interactions and improves absorption and translocation.”

Most foliar nutrients can generally be included within the tank mix when applying other plant protection products, although, as always, check label recommendations carefully, he adds.