Figuring out the cause of pitted pears

Stink bugs or stem-pitting virus? By Anna Mouton.

A new research project funded by Hortgro aims to establish the cause of dimpled and deformed pears seen in Western Cape orchards. The project is led by Dr Rachelle Bester and Prof. Hano Maree of the Department of Genetics at Stellenbosch University. “We are virologists,” explains Bester. “We are interested in whether a virus could be causing these symptoms.”

The pears look typical of a disease called pear stony pit which is thought to be caused by apple stem-pitting virus. The virus has been detected in symptomatic trees in two different South African studies in recent years. But some growers believe that stink bugs are the real culprits in damaging the pears.

A diverse team of Stellenbosch University researchers has been assembled to investigate both options. The group includes virologist Prof. Gerhard Pietersen of the Department of Genetics, entomologists Prof. Pia Addison and Steffan Hansen of the Department of Conservation Ecology and Entomology, and plant pathologist Dr Julia Meitz-Hopkins of the Department of Plant Pathology. Matthew Addison, manager of the crop protection programme at Hortgro Science, is also part of the group.

The virus hunters

The first step is to identify orchards that produce pitted pears. Known sites in Ceres and Elgin will be included. Affected trees will be monitored throughout the season for the development of damage. They will be checked for bugs and sampled for virus testing. This phase of the research is set to start this spring and cover three growing seasons.

“We are going to test the association between the symptoms and the virus,” says Bester. Affected orchards will also be followed to see whether the number of symptomatic trees increases — which could suggest spread of the disease.

Maree and Bester previously used a technique called high-throughput sequencing to screen plum trees with symptoms of marbling for agents of disease. This led to the discovery of a previously unknown viroid that has now been shown to cause plum marbling. They plan to use the same approach for pear trees.

Samples of trees with and without symptoms of pear stony pit will be collected later this year for analysis based on high-throughput sequencing. This will help unravel the association between pitted pears and apple stem-pitting virus as well as ensure that there are no novel disease agents lurking in the affected trees.

A moving target

Many viruses mutate readily — this helps them to evade the immune response of their hosts and to exploit opportunities to infect new hosts. Several international studies have shown that genetic variation is common in apple stem-pitting virus. Bester and Maree think that the South African situation is no exception.

They analysed apple stem-pitting virus from an orchard with pitted pears last season. “The variant of the virus that we found looks a little different from those that have been reported previously,” says Bester. “It may be that we need to relook at our detection methods to ensure that our current tests will also pick up this new variant.”

Apple stem-pitting virus is one of the diseases certified under the Deciduous Fruit Plant Improvement Scheme. Plant material is subjected to testing based on PCR — short for polymerase chain reaction. One objective of the new research project is to update the existing PCR test based on analysis of South African variants of apple stem-pitting virus.

Bester points out that the orchards where they have found apple stem-pitting virus are very old and may have been planted before routine testing was available. But, stresses Maree, this project will provide certainty that testing can reliably pick up infections.

Bitten by the bug?

Entomologists Hansen and Matthew Addison agree that bugs probably get blamed for more than their fair share of damage. “They’re a common insect,” says Hansen. “You can look at almost any plant and you’ll find them — that doesn’t mean that they’re causing damage.”

Two bug species have been recorded as pests of deciduous fruit in the Western Cape. One is the indigenous antestia bug — Antestiopsis thunbergii — and the other is the ubiquitous green stink bug — Nezara viridula — which is also known as the green vegetable bug.

Hansen has previously observed damage to a pear caused by an antestia bug. He points out that bugs as a group will cause different symptoms depending on the growth stage of the fruit and the species of bug. “You see a wide variety of symptoms. Sometimes it’s difficult to even recognise that it’s bug damage.”

Addison thinks that bugs are at most sporadic pests. “We’re turning up damage, but we don’t have a clue what causes it. Is there an organised monitoring process for bugs? No.” Monitoring for bugs is an integral part of the new project.

The researchers will also attempt to induce damage by placing bugs on fruit during the growing season. The bugs will be confined in mesh bags. Control fruit will be bagged to exclude bugs. The plan is to conduct these experiments in trees with and without apple stem-pitting virus.

Both pear stony pit and bug damage show up as dimpled fruit, and in both cases there may be a small scar at the base of the dimple. The area beneath the dimple is often brown and corky in bug damage. But bug damage tends to be superficial whereas pears with stony pit have multiple discoloured areas throughout the flesh. Stony pit lesions also typically have a gritty texture due to the formation of hardened cells.

“A main objective of this project is to see, if a bug feeds on a pear, does it cause a lesion? Or does it cause deformity?” says Bester. “The existing reports are so vague — we want to provide scientific proof that this disease is or is not caused by a bug.”

“But it could be that this is really hard to do,” cautions Maree, “and that’s why no one has managed to do it so far.”