Foliar sprays

Foliar sprays are useful for delivering fertilizers, fungicides, pesticides and PGR’s to plants.

Types of foliar sprays

Foliar sprays can be broadly categorized as either “non-systemic” or “systemic”.

“Non-systemic” sprays are used to treat problems located on the surface of the leaf. Most pesticides are non-systemic. Fungicides used to treat certain types of “surface” fungi such as mildews are also usually non-systemic.

“Systemic” spays are absorbed into the plant via ‘stomata’ and then transported via the vascular system to where they are needed (Fig 16.1). Foliar fertilizers, PGR’s and many fungicides are common examples of systemic sprays.

Wetting Agents

A wetting agent (or ‘wetter’) should always be added to foliar spray solutions. Wetters are a type of surfactant used to improve a spray’s capacity to “wet” foliage. They achieve this by lowering the surface tension of the foliar spray solution. With a lower surface tension, the spray droplets can collapse and spread evenly across the surface so that greater coverage occurs (Fig 16.3). Where systemic sprays are being used, the spray is able to cover more stomata which increases the opportunity for absorption (Fig 16.1).

In the absence of a wetter, large droplets will form on the leaves (Fig 16.2). These will either roll off the leaf quickly and be wasted, or may cause burning when intense light is present. Leaf burn is more common with relatively concentrated solutions.

Some leaves are harder to wet than others and therefore require a higher dose of wetter. For some ‘waxy’ leaves it is not possible to achieve complete wetting. In this case, be careful not to exceed the label’s recommended dose rate. There is a threshold concentration beyond which the wetting capacity does not increase. It is important to recognize this because excessive wetter concentrations can cause leaf burn.

Wetters also usually possess “hygroscopic” properties. This helps extend the duration that the foliar spray remains wet on the leaf. This is important because absorption of the active component only occurs while the spray remains wet.

Foliar fertilizers

Unlike roots, foliage is not adapted to absorb large amounts of fertilizer (nutrients). However, foliar spraying is able to take advantage of the significant combined 'surface area' of leaves and stems on a plant. This is defined in the following report by DR. H. B. Tukey (Dept of Horticulture, Michigan State College):

The leaf surface of a 12-year-old apple tree in Washington State is equivalent to one-tenth of an acre, even though that tree only occupies about one-hundredth of an acre. So there is a large feeding area”.

Consequently, when foliage is sprayed with a fertilizer formulation that can be easily absorbed, there is a large opportunity for nutrient input.

Sprays are mainly used for supplying nitrogen, iron and zinc. However, potassium and other trace elements can be absorbed through foliage. Radio active tests show that micro-nutrients, once sprayed, are in the sap stream within 1 hour. This means foliar sprays can be effective for quickly correcting certain nutrient deficiencies. They are also a useful supplement to root feeding when up-take is restricted because roots are diseased, damaged, or simply too small.

Specific foliar fertilizer formulations can be used to influence plant characteristics such as fruit set, fruit size and pest and disease resistance.

Best practice for foliar spraying

Without proper practice, foliar sprays can be wasted or cause problems such as leaf burn and mould. The following guidelines will help prevent the above mentioned problems from occurring:

1. The best time to spray is usually early morning, about 1 hour before daylight. This gives stomata sufficient time to absorb before light recommences. Spraying earlier than this risks mould growth because the foliage will be damp for a longer period of time.

In the early morning, humidity is also relatively high, temperature is relatively low and there is minimal light. Under these conditions the spray remains wet for longer so that maximum absorption can occur. In hot, dry conditions, the spray can dry too quickly and cause leaf burn and staining.

2.  For systemic sprays especially, never spray when the air temperature is above ~25°C (77°F). Apart from the concerns mentioned above, absorption at these temperatures is poor because in many species the stomata are generally closed.

3.  The spray device should be fitted with nozzles that produce a fine mist. This helps maximize surface coverage, especially on the underside of leaves where the majority of stomata are located. This is also important for non-systemic sprays such as pesticides because insects tend to harbour on the underside of foliage.

4.  Spray when wind is minimal. This is especially important with finely atomized sprays because they drift readily. If growing indoors ensure oscillating fans and ventilation units are switched off.

5.  Only lightly spray the leaves and stems with a thin film of moisture. There should be little or no run-off. Drenching the surface of the foliage is wasteful and can restrict the stomata’s ability to absorb.

6.  Use low salinity/ soft water. This will reduce the risk of leaf staining and burning.

7.  Where growing outdoors, delay spraying if rain is imminent. If rainfall occurs within 1 hour of spraying, re-spray within the next 1-2 days.

8.  Test compatibility before spraying the entire crop i.e. Test-spray a small patch of leaves and observe for at least 2 weeks.