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AHDB/NIAB EMR Tree Fruit Day 2021 – The Fruit Grower – Rob Saunders

This year’s Tree Fruit Day was of course, a webinar and was held over one afternoon, and so was even more convenient for busy growers ...

AHDB’s Scott Raffle reported that the numbers registered were even higher than they have been for this popular event in previous years, with attendees hailing from Plymouth, Inverness, and Northern Ireland. This is a positive outcome so in post-Covid times perhaps there will be a choice of attending virtually or physically.

Although there is a facility for a ‘Question and Answer’ session in the webinar format, the option to discuss matters with presenters and other attendees in the breaks is certainly missed.

Developing novel and alternative approaches to control pests and diseases

Scott Raffle kicked off the proceedings with a summary of the main results from the AHDB IPDM project TF 223 that sought to develop novel and alternative approaches to the control of some of the main diseases and insect pests affecting UK-grown tree fruit crops. The disease research focused on apple canker, apple powdery mildew and brown rot, and bacterial canker of cherry. Insect pest research included moth damage in apple, apple sawfly, apple fruit rhynchites, pear sucker, Anthonomus spilotus in pear, brown marmorated stink bug, and natural predation.

Additional work was done to survey new and emerging pests and diseases of orchard crops.

This was the first of new-style five-year projects that Rob Saunders AHDB Top Fruit Panel Chairman pointed out encompasses ‘a whole story’ and provides continuity with a longer span, which helps researchers make progress. There is a detailed handout to download from the AHDB website with guidance for growers resulting from this work under each pest or disease.

More updates for some of the pest and disease work were presented by researchers during the afternoon programme.

Rob Saunders, Hutchinsons agronomist and AHDB Top Fruit Panel Chairman

Disease control

Mat Papp-Rupar has been investigating whether we can harness endophytes to aid apple canker control. Rob Saunders describes this work as potentially a gamechanger for canker control. A tree is more than just one plant, a tree is a community of fungi, oomycetes, bacteria, insects, micro-fauna etc, which are living on and in the different organs – roots, shoots, leaves, and fruits. They can confer a whole spectrum of benefits and problems to the tree. Endophytes can reside within plant tissues and they enter plants from soil through roots, from air through leaves, and can be transmitted via seeds.

All of them interact with plants and some confer benefits such as growth promotion, stress tolerance, defence induction, and can become antagonists against pathogens. But some are pathogens or latent pathogens, and some. like saprophytes, are neutral. Mat has been looking at whether specific endophytes influence the canker susceptibility of apple trees, either directly as biocontrol agents or indirectly by inducing plant defence responses.

So, do endophyte communities differ in different apple cultivars? In a preliminary small-scale trial on four cultivars, two of which are resistant to canker – Golden Delicious and Grenadier, compared with two susceptible to canker – Gala and Braeburn, DNA sequencing was used to characterise endophyte communities. This revealed that trees from canker-resistant cultivars harbour different endophyte communities and canker-resistant cultivars had higher amounts of Epicoccum purpurescens (Ep) endophyte, a known fungal biocontrol agent against brown rot (Monilinia). In the laboratory, Ep showed biocontrol potential against apple canker. Can we introduce biocontrol edophytes into the the tree, and if so, do they protect against canker in field conditions?

In the latest field trial, where Ep and canker were co-inoculated into the leaf scars at 50% leaf fall in November 2019, promising results were found in summer 2020. The incidence of canker was up to 60% and Ep endophyte treatment reduced the canker infection rate by 50%. Mat says that this is an important result. The hope is that a specific strain of Ep for apple canker control could be developed and commercialised for use in the UK. Further development requires the involvement and investment of a company producing biocontrol agents. Rob Saunders commented that we are on the crux of a shift in thinking and “it’s an exciting time to be an agronomist”.

Sophia Bellamy, a CTP PhD student based at NIAB EMR, is investigating the biocontrol of brown rot in stone-fruit crops. There are three main pathogens causing brown rots, but Monilinia laxa is the worst in the UK. It can infect blossoms, twigs, and wounded and intact fruits of both cherry and plum. Sophia pointed out that chemical controls using fungicides are now limited and there are new resistant strains of the fungus. Physical control through the removal of mummified fruits is still important, but bacterium B91 or yeast Y126 offer the potential of biocontrol. In the laboratory spore germination of the fungus was compared after biocontrol agents were introduced. B91 worked well by producing inhibitory compounds and the yeast competed successfully with the fungus for nutrients and space. Brown rot latent infection is a particular problem where seemingly healthy fruits show disease after harvest.

Over the two years of her studies with B91 and Y126, Sophia has shown that, when applied to cherry fruit two weeks before harvest, they can significantly reduce post-harvest rot incidence in both cold storage and ambient temperatures. This is a promising result, but timing and isolating a specific strain of B91 for cherries is important for efficacy. Rob Saunders commented that using biocontrol measures is more difficult, but once we better understand the technology, they represent an exciting future and will help to manage residues near harvest.

The cause of apple replant disease (ARD), admits Professor Xiangming Xu of NIAB EMR, has been debated for years, but is now agreed to be mostly a pathogen problem. The relative dominance of the five major pathogens implicated varies with site. Rootstocks are known to differ in their response to ARD and the soil microbial community affects it.

Recent research has shown that it is possible that competitive interaction between oomycetes and fungal pathogens is causing ARD. Controlling all three ARD components (oomycetes, fungi, and nematodes) led to the best root development. Work on rotating rootstocks has given some interesting pointers and questions whether replanting in the alleyways may help establishment.

Certainly, Xiangming has been surprised how quickly – seven months – a new stable microbial community establishes in a newly planted orchard but we don’t know how much this influences tree growth.

A new EU-funded research project – Excalibur – on soil biodiversity and how it could help horticulture, includes trials looking at soil amendments in tree fruit. This work at NIAB EMR will focus on the use of different types of beneficial microbes to improve tree establishment. An orchard was planted last autumn with five rootstock genotypes and three specific soil amendments. Also, there will be detailed studies of root development in potted rootstocks.

The experiments on ARD and the resilience of the soil microbiome are being conducted jointly by CTP PhD student Chris Cook who introduced his work so far. A comparison of the bacterial and fungal populations found in soil from a grubbed organic orchard with those from a conventional orchard proved that they were very different. He will also be looking at the effects of waterlogging and drought on soil microbial populations.

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