A European perspective on post-harvest diseases in pome fruit. Q+A with Dr Marcel Wenneker

Dr Marcel Wenneker, senior crop-protection researcher at Wageningen University and Research in the Netherlands, shares his insights. By Anna Mouton.

Q: What is the status of post-harvest diseases in pome fruit in the Netherlands?

A: Post-harvest diseases are a minor problem on apples. Fibulorhizoctonia [lenticel spot] is an issue every five to eight years, but it depends how long the apples are going to be stored. If the apples are sold before February – so less than six months storage – it’s not a big problem.

In many countries, Neofabraea [bull’s-eye rot] is a problem, but our main apple cultivar – Elstar – is not susceptible, and our main pear cultivar – Conference – is also not susceptible. We do have incidents, but that’s in maybe one out of 1 000 orchards. In contrast, France, Italy, and Chile have a lot of problems with Neofabraea, because Pink Lady [Cripps Pink] and its relatives are susceptible to Neofabraea.

With pears it’s another story. When growers store pears longer than eight to nine months, they will most certainly run into problems with Cadophora [side rot]. This season some growers had 75% of the pears affected in July and August.

The issue is that Cadophora is present in the orchard, and growers cannot get rid of it. So now most of the growers know whether the disease is present, and that they have to dip those pears in pyrimethanil if they want to store them for a long period.

Q: What are some of the trends in the occurrence of post-harvest diseases?

A: The major post-harvest diseases in the Netherlands are different from those in other European countries, possibly because the other pear cultivars are less susceptible to Cadophora than Conference. Or possibly because most other countries have apples instead of pears. The Netherlands is really changing from an apple-producing country to a pear-producing country.

That worries me a little bit because of Cadophora, but also because of pear scab [Venturia pyrina] because we also have problems with pear scab in storage. We have a huge monoculture of Conference pears in the Netherlands. Five years ago, pear scab was a minor disease. Now we have a number of organic pear orchards where the growers can no longer control pear scab and from there it might go to conventional orchards as well.

Q: Do common post-harvest diseases such as Botrytis have an impact?

A: I am always amazed that most of the efforts in research go to Botrytis and Penicillium. Botrytis, Penicillium, and Monilinia are all mainly wound-related, so if you are picking carefully, Botrytis and so on shouldn’t be a problem. I had some trials where I did not spray for three years in a row. When we picked, we picked carefully, and we could store the fruit for twelve months with no disease problems other than latent diseases.

Nowadays in the Netherlands all pickers are very well-instructed. They have to use gloves, or they are not allowed to pick. When picking pears, you also need to do it properly, otherwise the stalk will break, and then it will puncture the other fruits, and then you will have a lot of problems.

Another advantage of the Netherlands is that our pack-store hygiene is very good, so Penicillium isn’t a problem. Our storage facilities are really specialised, and they do not make mistakes. Competition is fierce, so the moment something goes wrong, the storage facility will lose all their clients.

Q: What is the potential role of cultivar selection in reducing post-harvest diseases?

A: There hasn’t been any selection of cultivars for susceptibility to post-harvest diseases, but that’s now gaining interest because we also have problems with European fruit-tree canker [Neonectria ditissima]. It gives the tree canker and can also infect the fruit.

So for instance Kanzi [Nicoter] is susceptible to canker, and the fruits are also susceptible. And because Kanzi is a late variety, conditions are often wet at harvest, and at that time the cankers are sporulating, so the spores go into the stalk end. You can end up with 25%-30% of apples affected after a few weeks of storage.

Growers were unaware that Kanzi is susceptible to European fruit-tree canker. Then after two to four years it exploded.

Breeders look for production and quality, and sometimes for resistance to fire blight or scab. Scab-resistance is easy to test. If trees have the Vf gene, then it is okay. But for most of the post-harvest diseases there are no selection criteria.

Q: How do you approach the control of post-harvest diseases?

A: I think we need a more holistic approach, instead of focussing on one aspect, such as spraying. You need to know the life cycle of the fungus, how it infects, and if it is possible to do something after infection.

First you could have a look at the cultivar that you’re going to grow. Is it disease-resistant, and if it is disease-resistant, against what disease? Because, if you have a scab-resistant cultivar, and you do not spray against scab, then most likely you will not control a number of other diseases. So diseases that were not very relevant when you were spraying against scab might become relevant.

If the new cultivar isn’t disease-resistant, then the next question is whether the disease is present in your orchard. If it is present, what can you do to interfere with its life cycle? Where can you manage it? Is it possible to remove the inoculum sources? And if you cannot control the inoculum sources, what are the next steps?

For instance, spraying for Cadophora is pointless, because it is not very susceptible to fungicides. You also have the problem that the pears are in clusters. We did some measurements and the inside of the clusters had five times less product than the outside.

The other problem is that we don’t know when many of the post-harvest diseases infect. People often think that it is only late-season, but the diseases often already infect in June or July – that’s three months before harvest. I think the number of infection periods are really underestimated, and if we do not know the infection conditions, then growers are just spraying and hoping for the best.

Q: How do you prioritise research on the control of post-harvest diseases?

A: You need someone to do assessments in the pack houses. Before we did assessments, we had these big posters with 25 different diseases and the recommendation was that you have to use chemicals against 25 diseases.

When we did assessments, it turned out that we had two or three real problems, and that we should focus on those problems. We need to pay a lot more attention to post-harvest diseases, because if we pay attention to the proper diseases, then maybe we can more easily manage the problems, instead of just spraying, spraying, spraying.