The practicalities of effective greenhouse hygiene

The intensive but highly productive practice of growing crops in greenhouse environments lends itself to significant pest and disease threats. Paddy de Vries of Intense Agri Hydro provides valuable insights on dealing effectively with these.

The practicalities of effective greenhouse hygiene
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The proverb ‘Cleanliness is next to godliness’ is frequently used by Paddy de Vries when describing the importance of maintaining a high standard of hygiene in an intensive cropping enterprise.

Too often in South Africa, he says, these enterprises pay only cursory attention to managing the biosecurity aspects that are so crucial for minimising pest and disease risks.

By contrast, the interiors of many greenhouses in the Netherlands, where intensive crop production is an extremely large and highly lucrative industry, “resemble sterile laboratory environments, so afraid are the Dutch of their intensive crops being infected with pests and diseases”, says De Vries.

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READ MORE Greenhouse production in SA – learning from Europe 

“Maintaining effective biosecurity in greenhouses is not easy, and can add to production costs, but it is essential. And it needs to be implemented in South Africa on a far larger scale.”

He warns that potentially devastating attacks by pests and diseases in intensive cropping environments are often insidious, as they are overlooked until the problem has slowly grown to such an extent that it is almost impossible to remedy without financially crippling the enterprise.

South Africa’s intensive cropping industry should not be too proud to take a leaf out of the biosecurity and hygiene management of similar industries in other countries such as the Netherlands and China, according to De Vries.

“There are collectively over 2,8 million hectares of intensively grown crops in China. This is far larger than the South African industry, which is in its infancy phase with a mere 1 500ha to 2 000ha. We intensive croppers in South Africa should be prepared to change our mindsets on a number of aspects related to biosecurity and hygiene management,” he notes.

For example, the practice of keeping greenhouses sealed as tightly as possible against pests and diseases is slowly losing ground to greenhouses where the roofs can be fully opened when required.

Interestingly, continues De Vries, even though pests and diseases can enter these greenhouses via this route, the crops growing inside are often so healthy that they do not transmit the stress signals that typically attract these threats!

Improving production
It is vital that South Africa’s intensive cropping operations become as productive and cost-effective as possible in order to survive, says De Vries.

Research shows that the country’s intensive croppers achieve an average annual increase of only 1% on the value of their crops. While ever-increasing production costs, which are mostly beyond growers’ control, are largely responsible for this, it is still possible for South Africa’s intensive croppers to improve production and yield in order to benefit from reduced production costs per unit sold, he says.

“Never allow production to stagnate or decrease,” he warns. “Don’t be left behind when it comes to continually increasing yield per square metre.”

He concedes that there always seem to be new, more vigorous stronger pests and diseases threatening intensive crop production, and fewer chemicals that remain legal for use against existing and new pests and diseases. But there are other ways to manage these threats to achieve a profitable crop yield.

De Vries quotes Dr William Albrecht: “Insects and disease are the symptoms of a failing crop, not the cause of it.”

“It’s not the overpowering invader we must fear but the weakened condition of the victim,” he continues.

He stresses that water quality in South Africa is a major problem and this adds a challenge to keeping crops healthy.

“Fortunately, there are treatments that intensive growers can use to treat this water to acceptable quality for irrigation or hydroponic purposes.”

Positive balance

Many aspects of crop health management are interconnected. If one aspect is neglected, it is likely to have a negative knock-on effect. A greenhouse manager should aim to achieve a positive balance across these factors so that crops are healthy enough to best resist pests and diseases.

According to De Vries, research shows that plants containing low sap Brix levels and low pH (more acidicity) are generally more susceptible to attacks by pests and diseases.

‘Brix’, he explains, does not specifically refer to the sweetness of a plant’s tissue; it shows total dissolved solids, which include sucrose, starch, organic acids, amino acids, soluble minerals, and others. Typically measured in-field using a refractometer, plant-sap Brix should, in general, be kept greater than six.

“To calculate an accurate trend in plant Brix levels, these levels should be measured and recorded at the same time every day,” he advises.

Increasing electrical conductivity in a plant’s root zone will not necessarily increase the plant’s Brix. Instead, a farmer should aim to improve the plant’s nutrient uptake and metabolism of minerals.

Increasing photosynthesis in plants is also essential for improving Brix.

Potassium (K), calcium (Ca) and other positively charged cations are key nutrients for increasing Brix in plants. Applying silicates may help increase total dissolved solids in plants. Silicic acid, explains De Vries, is widely used by intensive cropping operations in the Netherlands. Silicates also provide added protection against pests and diseases.

Salicylic acid, similar to aspirin, is another important product that helps plants improve their defences against pests and diseases.

Acidity and alkalinity
De Vries points out that plant pH is not related to what plants are fed or their growing medium. Therefore, using an alkaline feed or plant growth medium will not increase plant pH; neither will acidic feeds or plant growth mediums lower plant pH.

It is Ca and magnesium (Mg) that regulate plant pH. Making Ca and Mg more bio-available to the plants is crucial (although it is still a major challenge to get Ca to move easily throughout all parts of a plant).

De Vries stresses that it is essential to understand the ideal ratios between various minerals so that an excess of one does not inhibit the plant’s access to another. For example, too much K in a plant can hamper the movement of Ca.

READ MORE Managing no-till soil acidity and fertiliser requirements

Mulder’s Chart, which shows nutrient interactions, is a crucial reference for any crop farmer, he says. An excess or imbalance of ammonium, for example, predisposes a plant to infection by diseases such as Fusarium spp and anthracnose.

“This excess or imbalance also encourages attacks by damage-causing nematodes, and reduces plant uptake of calcium, magnesium and potassium,” he explains. “Potassium silicates can be used to increase a plant’s total dissolved solids and strengthen its cell walls against attacks by pests and diseases, such as powdery mildew.”

Numerous ways are available to reduce the introduction and spread of pests and diseases in an intensive cropping enterprise, yet many of these are overlooked, says De Vries.

Options include hand and shoe sanitisers, regular and effective disinfection of tools and machinery and strict movement control of people and equipment into – and within – the enterprise.

However, such protocols are useless if they are not consistently enforced and monitored, he adds. Greenhouse managers should personally and regularly inspect these to ensure that they are fully functional and being used correctly.

“Also take control of washing, disinfecting, issuing and collecting protective clothing used by greenhouse employees and management. This clothing must not be allowed to leave the premises because it can bring back numerous pests and diseases.”

He stresses the need for strict and effective protocols to identify and remove diseased or dead plant material. Too often diseased and pest-carrying plant material is carried through healthy sections of the intensive cropping enterprise, with costly results.

When disease or pests are detected in an intensive cropping enterprise, the exact nature and extent of the problem must be quickly identified. It is unnecessary to remove numerous plants when only one or two are infected.

Treatments are available to apply to lesions caused by certain diseases such as Botrytis spp, without having to remove the whole plant. Conversely, it may become necessary to clean out and disinfect an entire greenhouse if too many plants are infected.

In short, an intensive crop producer must know and understand the various common pests and diseases in order to select and utilise the most cost-effective controls.

De Vries recommends that effective drainage be installed in areas where standing water accumulates. These can be breeding grounds for a variety of diseases and vector insect pests, such as fungus gnats and shore flies.

Maintain good records
Finally, detailed records must be kept of pest and disease challenges, the damage they caused and how they were controlled. Such records can be used to formulate better preventative measures against possible future attacks and develop better early detection and control measures.

Based on a presentation given at the recent Intensive Growers’ Association’s Spring-Summer 2016 Symposium held at the Cedara Agricultural Research Centre, in KwaZulu-Natal. Phone Paddy de Vries on 083 633 1222, or email
[email protected]. Visit intensagri.co.za.

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Lloyd Phillips joined Farmer’s Weekly in January 2003 and is now a Senior Journalist with the publication. He spent most of his childhood on a Zululand sugarcane farm where he learned to speak fluent Zulu. After matriculating in 1993, Lloyd dreamed of working as a nature conservationist. Life’s vagaries, however, had different plans for him and Lloyd ended up sampling various jobs in South African agriculture before becoming a proud member of the Farmer’s Weekly team.