Under, On and In Grapevines: Vineyard Ecosystems

MacDiarmid R1, Arnold N1, Avila G1, Bell V1, Blouin A1, 2, Clothier B1, Cole L1, Cosic J2, Fedrizzi B2, Gentile R1, Giraldo-Perez P2, Goddard M2, Grab F1, Green S1, Greven M1, Jesson L1, Klaere S2, Malone L1, Mason K1, Mundy D1, Sandanayaka M1, Sorensen I1, Raw V1, Taylor T1, van den Dijssel C1, Vanga B1, Wood P1, Woolley B1.

1 Plant & Food Research. 2 The University of Auckland.

As we reported in the previous Research Supplement, we have now passed the Year Two milestone of the Vineyard Ecosystems programme, a seven-year research programme developed by New Zealand Winegrowers and co-funded by the Ministry of Business, Innovation & Employment (MBIE). The programme investigates what is “Under, On and In Grapevines” to create a new knowledge network that reveals interactions of vineyard practices with the vineyard ecosystem, including vines, groundcover, soil, soil microbial life, invertebrate vine pests (including pathogen vectors) and pathogens over time.

The research focuses on 12 white and 12 red cultivar study blocks on commercial vineyards split equally between Marlborough (Sauvignon blanc and Pinot noir) and Hawke’s Bay (Sauvignon blanc and Merlot). Half of the cultivars and blocks in each region are under “Contemporary” and half under “Future” management. “Contemporary” management consists of semi-permanent inter-row groundcover and spraying herbicide to maintain a bare soil under-vine strip. Additionally, synthetic fungicides and pesticides are used to combat fungal diseases and insect pests, and synthetic fertilisers are used to mitigate nutritional deficiencies. “Future” management consists of a semi-permanent groundcover (inter-row and under-vine) comprising a wide range of plant species (no herbicide is used, but management may include inter-row and/or under-vine soil cultivation). Naturally occurring products (if required) are, by preference, used to combat fungal diseases and insect pests.

In Part 1 of our report, we summarised findings for Research Aim 1.1, “The vineyard as an ecosystem”. Part 2 focuses on Research Aim 1.2, “Relating under-vine management, biota and leafroll virus”,  and Research Aim 1.3, “Pathogen management”.

Although this article summarises our annual assessments, it should be stressed that the full value of the data collected and analysed in this programme will only be realised over a multi-year timeframe. Annual results may – or may not – be indicative of long-term trends and correlations.

 

Research Aim 1.2: “Relating Under-Vine Management, Biota And Leafroll Virus”

Research Aim 1.2 brings together several research projects on biota, bugs, and leafroll virus. In this research we are directly asking “How does UNDER-vine management impact what is ON and IN the vine?”

Under and On Grapevines

Successful management of leafroll virus currently relies on adopting multiple tactics, including identifying and roguing (removing) virus-infected vines and controlling mealybugs, the insect vectors that transmit the disease. To complement these approaches, we assess whether a persistent cover of plants in the under-vine zone can attract mealybugs away from the vines. By minimising the extent to which mealybugs interact with the vines it may be possible to reduce the influence of leafroll virus and increase vine longevity. For this research, we are using eight Hawke’s Bay vineyard study blocks planted in mature Merlot vines of which five blocks are managed under the Future regime and three under the Contemporary regime. Based on the results of the work, grower recommendations for groundcover management will be developed.

Where are the mealybug – on grapevines and/or on groundcover plants? To better understand mealybug habitat selection, and the extent to which it might alter over time, assessments were undertaken on each of three occasions between October and April in each of 2015/16 and 2016/17. Vaughn Bell and Tara Taylor in Hawke’s Bay (assisted by Victoria Raw with Franzi Grab in Marlborough) used pheromone-baited traps to determine the presence of longtailed and citrophilus mealybugs in all study blocks. Numerically, the citrophilus mealybug dominated in all blocks, representing greater than 95% of all the mealybugs observed. By also looking early, mid and late season for mealybugs on vine leaves (200 leaves per block, per visit) and selected groundcover plants (50 plants per block, per visit), we sought to determine the vineyard habitat from which male mealybugs caught in the pheromone traps originated.

Mealybugs were rarely found on vine leaves or on groundcover plants in November in any block in either growing season. However, from January and into March, numbers of mealybugs typically increased but the numbers found varied widely between blocks. In two Future and two Contemporary blocks, numbers of mealybugs on vine leaves were consistently low in both growing seasons (range: 0-13 mealybugs per 100 leaves inspected). Over the same period, mealybugs were generally found on less than 10% of groundcover plants. By contrast, in another three Future blocks, the March vine inspections yielded many mealybugs (range: 26-106 per 100 leaves inspected), with more than 50% of groundcover plants infested from January. In the remaining Contemporary block, a blowout in the mealybug population in March 2016 (450 per 100 leaves inspected) was largely reversed 12 months later (24 per 100 leaves inspected). Crucially, this block remained a notable exception in this study in that there was a scarcity of groundcover plants and poor plant species richness, with inspections of this habitat revealing very few mealybugs.

Which groundcover plants hosted mealybugs? Since this study started, a total of 2,400 groundcover plants, comprising 11 species, were collected from the eight study blocks. Every plant collected was individually inspected for mealybugs. White clover and hawksbeard were the host plants most likely to be found with mealybugs with 41 and 32% of white clover plants and 37 and 35% of hawksbeard for the 2016 and 2017 years, respectively. With mealybugs present on plants throughout each growing season, both white clover and hawksbeard are clearly important vineyard groundcover hosts that are arguably more attractive to mealybugs than the vines themselves.

What impact does under-vine management have on groundcover species? Habitat use by mealybugs can be influenced by factors such as herbicide use or soil cultivation. If either practice is implemented in the under-vine or inter-row zones it will destabilise the treated area, thereby reducing its suitability to mealybugs. To assess the extent to which either zone was affected by management decisions in the eight study blocks, we estimated the groundcover composition. In each block during both growing seasons, a 0.25 m² quadrat was placed in nine pre-determined positions in the inter-row and the same number in the under-vine zone. This process was repeated three times during each growing season, in the same quadrat.

In all blocks, bare ground and/or grass were dominant features in both zones throughout the growing season. Notably, neither habitat can support vineyard mealybugs. Multiple groundcover plant species were recorded but their presence and the frequency of detection varied widely by block and with the time of the growing season. Of the species known to support mealybugs, white clover was commonly found in all the Future blocks, irrespective of the inspection date; it was not present in the Contemporary blocks. Hawksbeard was also commonly detected in all five Future blocks but only in two Contemporary blocks and then, only infrequently.

In Grapevines

Does groundcover impact leafroll incidence? If groundcover habitat is available and remains relatively undisturbed, it could reduce grapevine susceptibility to mealybugs. Any reduction in the vine/vector association will reduce the potential for leafroll virus spread. Hence, in April 2016 and April 2017, virus incidence was also assessed in the eight study blocks. By looking for persistent changes to foliar colour and form, we identified the numbers of virus-infected Merlot vines in each block and plotted their position within the vine row. In six of the eight blocks, annual virus incidence was low (less than 0.5%) in both growing seasons. In the last two blocks, annual virus incidence was higher, ranging from 9.6 to 19.7% in 2016. By 2017, roguing of all the 2016 infected vines in one block saw annual virus incidence reduced to 3.2%, whereas in the other block, the absence of roguing resulted in cumulative infections increasing to 21.3%. While data collection is underway, it will be another one or two growing seasons before we can begin to assess the extent to which vineyard groundcover might influence leafroll virus.

In summary, there is no evidence to date that the numbers of mealybugs found in the Merlot study blocks can be characterised simply on the basis of a Future or Contemporary management system. Where evidence of a potential split based on the management might occur was in relation to plant species present. White clover and hawksbeard, which were frequently found with mealybugs, were universally present in Future study blocks but they were essentially absent in all of the Contemporary blocks.

On Grapevines; Mealybug Biological Control

Biological control is a natural process for controlling pests by using other living organisms. In horticulture, pest insects are attacked by another group of insects collectively known as natural enemies or beneficial insects. In New Zealand vineyards, the most widespread and the most important insect pests are the citrophilus and longtailed mealybugs. Their natural enemies include predators like ladybirds, together with multiple species of tiny wasps, known as parasitoids. With parasitoids arguably the most important mealybug natural enemies, they are the focus of this study.

In this long-term study, we aim to improve our understanding of those parasitoid species likely to be exerting the most influence over vineyard mealybugs. By distributing plastic mesh bags baited with seed potatoes supporting laboratory-reared colonies of either the citrophilus or longtailed mealybugs, we expect to identify parasitoids in differently managed vineyards. So far, more than 880 parasitoids have been recovered. The most commonly found parasitoid species was Coccophagus gurneyi, which was mainly retrieved from citrophilus mealybugs deployed in the Future study blocks. To date, few parasitoids have emerged from mealybugs deployed in the Contemporary blocks, nor was there much evidence of parasitism of longtailed mealybugs in either the Future or Contemporary vineyards. Understanding the impacts of vineyard management on mealybug parasitoid populations is likely to guide the development of grower guidelines that support mealybug biological control.

Under Grapevines – Grass Grub

Grass grubs are insect pests found in some Marlborough vineyards. Damage by adult beetles is primarily confined to feeding on vine foliage in spring, with larval feeding on roots implicated in the decline of young vines. The current synthetic pyrethroid insecticides used are toxic to beneficial insects that preferentially attack other pests, such as mealybugs. Alternative management options that are less toxic to beneficial organisms could be of value to Contemporary and Future vineyard systems. Therefore, we are assessing a biological insecticide, Invade™, for its ability to infect grass grubs with a naturally occurring but relatively rare soil-dwelling bacterium, Serratia entomophila to cause amber disease. This disease causes grass grub larvae to stop feeding within 2 – 3 days and eventually starve, preventing development to the adult stage. Crucially, Invade has the potential to be conveniently delivered to the target areas by way of drip irrigation lines. In conjunction with AgResearch, pre-screening of two Marlborough vineyards has shown that no amber disease was present naturally. Invade was watered onto plots in these vineyards and grass grubs later sampled from the treated areas were found to have been infected with amber disease, while all larvae from untreated areas were uninfected. This experiment is showing promising results. Further sampling at the end of the 2018 season will demonstrate whether the disease has persisted in the grass grub population

 

Research Aim 1.3: “Pathogen Management”.

In Grapevines

The focus of the third research aim is very much “Inside the grapevine”. Dion Mundy, Bex Woolley, Arnaud Blouin, Robin MacDiarmid, Bhanupratap Vanga, and Simon Bulman from PFR are identifying what microbes and viruses are present and which of them are detrimental or beneficial to the vine. The research addresses the overarching question, “What factors correlate with vine health in the presence of detectable pathogen burden?”

To address this question, a non-destructive grapevine trunk disease (GTD) assay has been developed by Dion and his team to detect the presence of pathogens using next generation sequencing (NGS). The effectiveness of this method is being continually evaluated against traditional pathogen growth assays in culture plates. Doctoral candidate Arnaud Blouin has developed and optimised a world-leading method to enrich RNA for those molecules encoding viruses. With these new methods in hand, the bacteria, fungi, and viruses inside vines are being identified, regardless of whether the organisms are good or bad for the plant, or whether they can be cultivated in the laboratory.

Using these methods we have identified two pathogens that had not previously been viewed as important in grapevines. Phaeomoniella chlamydospora and Cadophora luteo-olivace have previously been recorded in New Zealand but have not been high on the priority for research in our mature trunk-diseased grapevines. We will be keeping watchful eyes on the correlation of these pathogens with trunk disease expression. We have also identified one potentially beneficial fungus, Aureobasidium pullulans, which lives in the vine and may play a role in ameliorating GTD symptoms. The association of this organism and the health status of grapevines, in the presence of GTD pathogens, will be tracked to determine the strength of any beneficial association.

A possibility exists that grapevine rupestris stem pitting virus (GRSPV) is a beneficial virus; with transmission only known to be by graft union, GRSPV has been selected as a common virus infecting grapevines in New Zealand and internationally. While developing the virus detection methodology, it was used as a positive control for the detection of other viruses in grapevines. The health status of individual grapevines infected with GRSPV compared with GRSPV negative plants should provide some information on its benefits to grapevines in New Zealand.

The information generated from data gained from “Inside the vines” is being collated in the Disease Risk Register (for pathogens) and the Grapevine Microbiome Register (for all microbes). The ultimate aim of RA1.3 is to deliver one or more novel methods to manage terminal diseases of grapevines, such as trunk disease and leafroll virus infection. Already, the project has identified promising organisms that may be useful in disease treatment. We will be observing the performance of vines closely through the Vineyard Longevity Predictive Model to note whether any potentially beneficial organisms do indeed influence plant health in the presence of a pathogen.

Conclusion

The focus, scale and duration of the Vineyard Ecosystems programme is, to the best of our knowledge, unique in grape research. Where other research has focused on the interactions between two or three variables, we consider interactions between five variables “Under, On and In Grapevines”:

  1. Plant
  2. Environment (both geography / weather)
  3. Microbiome
  4. Management methods
  5. Changes over time

Knowledge of the microbial ecosystem “Under, On and In Grapevines” is already being expanded through this programme, creating new insights into “microbial terroir” including pathogens that may cause grapevine diseases or that may alleviate those symptoms if used effectively in the future. By recording the range of organisms that is present, and those risk organisms that are not present, this information strengthens the biosecurity status of New Zealand vineyards. By informing growers involved in the trials about the incidence of trunk diseases and leafroll virus they immediately benefit as they may act to manage or rogue infected vines.

Over the multi-year time-frame of the programme, trends are likely to become apparent and correlations between the presence of pathogens and potentially beneficial organisms or management systems, soil properties, and weather may be discerned. Experimental data generated by this research will provide evidence-based solutions to threats to grapevine longevity, which will improve the environmental and economic impacts of grape growing in New Zealand.

This article first appeared in the February / March 2018 issue of the New Zealand Winegrower magazine.