Back to earth

New crop protection projects focus on orchard ecology. By Anna Mouton.

There’s an old saying that nature abhors a vacuum, and this certainly applies to orchards. Establish a block of fruit trees and other life follows: plants of one kind or another colonise bare soil. Insects — some of them pests — take up residence. Bacteria and fungi grow everywhere. What was intended as a monoculture soon becomes an ecosystem.

How should growers view this proliferation of species in their orchards? The traditional response was to take up arms — mostly chemical — to protect tree health. But, argues Matthew Addison, programme manager for crop protection at Hortgro Science, it’s better to have a diverse orchard with minor sporadic problems than no diversity.

“With no diversity you have chronic problems that cost you a lot of money,” says Addison. He explains how three new research projects aim to better understand how orchard ecology interacts with soil condition and tree health.

Fungal communities and canker

Chipping an old orchard to mulch a new one might seem like a great idea. But is it a disease risk? “I think it is,” says Addison. “I’m worried that you bring in wood-rot fungi that’s been sprayed for forty years with fungicide and it then establishes.”

Fungi that cause wood-rot and cankers can infect young trees leading to die-back and death. Anecdotal accounts suggest that die-back in new orchards is increasing. Could this be related to chipping old orchards for mulch? No one knows. We also don’t know if harmful fungi can be spread by other forms of mulch such as pine chips or straw.

A project led by Prof. Lizel Mostert from the Department of Plant Pathology at Stellenbosch University seeks to answer these questions. The study will identify the fungi associated with wood rots and cankers and develop better testing methods for them. These new testing methods will be used to study the occurrence and spread of harmful fungi in the orchard.

“We also need to determine the effect of using straw or pine chips on the orchard floor and soil ecology,” says Addison. “Will we import pathogens? Chipping and mulching with apple wood is by far the highest risk but there are others.”

Counting on collembola

Collembola — also called springtails — are minuscule insect-like creatures that mostly live on detritus. Every grower should care about collembola because these tiny organisms help to keep soils healthy and productive and are important food for spiders and predatory mites. There are more than a hundred known species of collembola in South Africa and research has shown their potential as indicators of biodiversity and soil health.

A new project will gather data on collembola in orchards and vineyards subject to different interventions. These include four types of cover crop as well as no cover crop. Plantings with and without shade net will also be compared. Dr Charlene Janion-Scheepers, an expert on collembola at the University of Cape Town, is leading the study.

“It feeds into, what are we doing on our orchard floor? How important is it?” says Addison. He believes that a healthy litter layer filled with collembola contributes carbon to the soil and provides shelter and food for beneficial organisms such as predatory mites. The complex interactions between these mini-beasts and cover crops can ultimately bring about a more stable orchard ecosystem with fewer pest outbreaks.

Mites make right

Mites live everywhere from your pillow to your eyelashes to your orchard — there are nearly 50 000 species described globally. A project led by Prof. John Terblanche of the Department of Conservation Ecology and Entomology at Stellenbosch University will examine how pest and predatory mites fit into the orchard ecosystem.

The researchers will use a combination of approaches. Mites will be tested in the laboratory to see which prey they prefer. Mites and potential prey species — such as collembola — will also be sampled from orchards and their body tissues examined using a technique called stable isotope analysis. This will provide information about diet across seasons and from different orchards. Understanding who eats who on the orchard floor is critical to maintaining a healthy orchard.

“Is the grass litter important or is it broad-leaved plants? Are the predatory mites eating collembola?” wonders Addison. Predatory mites usually do a good job of keeping the pest species under control. The question is how to boost the predators without amplifying the plant-eaters.

Biological control is set to become ever more important if growers want to access high-value markets. These new crop protection projects are another step in helping the industry reduce its dependency on chemicals while enhancing sustainability and resilience.

Image of collembolan supplied by Andy Murray | A Chaos of Delight.