Tree vigour is one of the major challenges of orchard intensification. Without controlling vigour in high-density orchards trees rapidly outgrow their space and canopy crowding can cause yield decline. The National Intensification in Tree Crops Project (AS18000) aims to explore different ways to control vigour in mangoes and help improve the success of high-density orchards.
Vigour is essentially growth. An example of different vigour is comparing different varieties, such as the difference in growth potential between a Kensington Pride and a Keitt tree. Vigour can, however, be affected by a large range of factors.
Mango trees require some vigour to grow the tree and renew a healthy canopy to support the crop. However, too much vigorous vegetative growth can compete for resources with flowering and fruit development, reducing tree yields, and increasing tree size. Larger trees require high levels of pruning, increasing the cost of maintenance. To control vigour, growers will also often restrict mango production to poorer soils, tightly control nutrition inputs and timing, and impose irrigation related stress on trees.
In high-density, intensive orchards, high-vigour trees rapidly outgrow their allocated space leading to canopy crowding and yield decline. As part of the project our team has been exploring different ways to control vigour in mangoes to reduce canopy crowding and maintain productivity in smaller, more densely planted trees. Below we discuss the major methods of managing mango tree vigour.
Pruning is currently the main management technique used to control the excessive vigour of mangoes. Pruning can reduce tree size and open up the canopy to allow light and spray penetration, however pruning on its own is unable to sustainably maintain trees small enough for high density orchards without negatively affecting yields. This is especially true for higher vigour varieties. One of the effects of heavy pruning is stimulation of vegetative growth at the expense of plant carbohydrate reserves that reduces availability for flowering and fruit set.
Variety or tree genetics has the greatest influence on tree vigour. As pointed out earlier in this article; the vigour of varieties such as Kensington Pride and Keitt can differ considerably.
Some high-vigour varieties are not suitable for high-density orchard systems. Low-vigour, but high-productivity, varieties are preferred for intensive production systems as they require less pruning while still fruiting and flowering strongly. Low-vigour, high-productivity trees are also a major focus of the Queensland Department of Agriculture and Fisheries’ (DAF) mango breeding program.
Rootstocks that induce low-vigour and high-productivity are a tool used to successfully reduce tree vigour in temperate tree crops, such as apples. Rootstock development has accelerated the transition to high density production systems in these crops. In mango, the most common rootstock used in the industry is Kensington Pride, which is chosen for its availability and genetic uniformity, rather than inducing any desirable characteristics in scions.
One of the major components of the project is to explore and select vigour reducing rootstock varieties for mango. The first phase of this research screened over 90 rootstock varieties for their effects on vigour and productivity on two scion varieties. Four of the most promising rootstocks have been advanced to second phase field trials, where their effects will be evaluated on a greater number of scion varieties in different growing regions. These regional evaluation trials will evaluate if the rootstock performance is consistent across varieties, soil types, climate zones and management systems.
The drawback to rootstock development, like any varietal development, is that development and evaluation takes time, and it will be at least five years before we can determine the effectiveness of lowvigour, high-productivity rootstocks for Australian mango producers.
The arrangement or architecture of the branches in a tree not only form a scaffold to hold leaves and fruit but can also influence tree vigour. While branch architecture is mainly dictated by the variety, it can also be influenced by tree training and pruning.
The project research team is investigating novel tree training and branch architectures that reduce tree vigour. One of these systems is the espalier training system, shown in the photos in this article, which is common in temperate crops. Branches are bent, either horizontal or below, by fastening them onto a wire, while retaining a central leader. This training system has kept mango trees smaller than the conventionally trained and pruned trees for the first eight years of this trial. Tree vigour and productivity will continue to be monitored as the orchard ages.
PLANT GROWTH REGULATORS (PGRS)
All growth events (vegetative flushes and flowering) in a tree involve plant signaling compounds (hormones) for their initiation and growth. PGRs are chemical products that can enhance or
block this signaling. PGRs have been used in many crops to influence vigour and other plant functions.
Paclobutrazol, a common PGR, has been used as a soil drench in mangoes for several decades to increase flowering. Current usage only minimally effects tree vigour, though its use for vigour management is being explored. The effect of PGR’s can be variable and are typically influenced by variety, tree age, soil type, application timing, nutrition and water availability.
The DAF mango research team have begun investigating the effects of other PGRs on mango growth to determine their potential use in the management of vigour and productivity.
GROWING INTO THE FUTURE
The project research on mango tree vigour is leading to more vigour management options that don’t compromise production. These options will reduce some of the current vigour barriers to high-density mango production systems, enabling productivity gains to be more easily realised. It is likely that a combination of vigour management tools will be required to sustainably maintain small trees suitable for intensive mango production. Ultimately, successful management of excessive tree vigour will reduce labor and input costs while increasing productivity.
This article is the second in a series from the AS18000 project outlining some of the key orchard components. If you missed the earlier article on light in the orchard, you can find it the October 2021 (Spring) edition of Mango Matters.