Make more of micronutrients, Hutchinsons agronomist advises

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.

Boron

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

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

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

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

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

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

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.

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