What is pear stony pit?

Conflicting views on a century-old disease. By Anna Mouton.

Pear stony pit is a disease of pear trees that shows up as deformed fruit. The fruit are dimpled or pitted. Severely affected fruit may have a distorted shape and become gritty and difficult to cut. So-called stone or grit cells in the flesh together with the hardness of the fruit give the disease its name.

Signs of pear stony pit seem to only occur in the fruit of certain cultivars — symptoms have been reported in South African orchards of Beurré Bosc and Forelle. Beurré Bosc trees can also develop cracking and shrinking of the bark. Affected trees may have less foliage and are more susceptible to stress.

Several pear cultivars can develop fruit abnormalities while others may have the disease without showing signs. Pear stony pit sometimes gives rise to yellowing of leaves along smaller veins. Mottling of leaves may also occur.

Pear stony pit was described from Oregon and Washington State more than a century ago. The cause was and is still thought to be a virus. Pear stony pit was first reported from South African orchards in 2015 by Emcee Gagiano. Gagiano found apple stem-pitting virus in four pear trees that showed symptoms of pear stony pit. The trees were sampled in Grabouw.

Does that mean that pear stony pit is present in South Africa and is caused by apple stem-pitting virus? Not necessarily. Gagiano also found apple stem-pitting virus in seven pear trees that did not have symptoms. And some growers believe that bugs — not a virus — are to blame for deformities in South African pears.

Apple stem-pitting virus

Apple stem-pitting virus infects pome fruit around the world. It is associated with a range of disorders including growth reduction and production losses — but surveys have shown that the virus can often be present without causing any signs of disease.

Researchers in several countries have demonstrated a relationship between apple stem-pitting virus infection and symptoms of pear stony pit. A recent study in India sampled trees with and without pear stony pit and found apple stem-pitting virus in the symptomatic trees only. The normal trees tested negative.

The problem with these studies is that association does not prove causation. The link between a disease and its cause should ideally be proven by fulfilling Koch’s postulates. This would involve isolating apple stem-pitting virus from trees with pear stony pit and using the virus to infect trees that are healthy. The healthy trees should in turn develop pear stony pit. The final step is to complete the cycle by again isolating the exact same apple stem-pitting virus from the newly symptomatic trees.

Fulfilling Koch’s postulates is not always possible in practice. Scientists often have to rely on correlation to try and find the cause of a disease.

Preventing pear stony pit

Transmission of pear stony pit by infected plant material was demonstrated back in 1938. Previously healthy Beurré Bosc and Beurré d’Anjou trees became symptomatic after grafting with buds from diseased Beurré Bosc trees. Nursery trees that had been created with infected material were considered the source of the disease in orchards.

The spread of pear stony pit through infected plant material has since been confirmed many times. Infected trees cannot be treated, and planting uninfected trees is the only way to prevent the disease. There is no evidence that pear stony pit is spread by insects or other vectors.

Plant material in pome-fruit nucleus units is tested for apple stem-pitting virus under the Deciduous Fruit Plant Improvement Scheme, but apple stem-pitting virus was detected in pear orchards within the past year. The use of uncertified plant material may explain why this virus persists in South African orchards.

A test based on PCR — short for polymerase chain reaction — is used to detect the presence of the viral genome for purposes of certification. Ongoing mutations in viruses can sometimes lead to a previously accurate test becoming unreliable. Apple stem-pitting virus is known to have genetic variability, so a new project funded by Hortgro will look into whether the current PCR-based test needs to be updated.

Plant-feeding bugs

Bugs are insects in the order Hemiptera and include aphids, planthoppers, leafhoppers, and grain chinch bugs. Bugs have piercing and sucking mouthparts.

True or typical bugs are in the suborder Heteroptera. This is the group suspected of causing dimpling in fruit.

Growers often refer to dimpling as stink-bug damage. Stink bugs or shield bugs include the pentatomid bugs. Several stink bugs are important agricultural pests. One example is the brown marmorated stink bug — Halyomorpha halys — which has caused losses of tens of millions of dollars in apples and other crops. It was introduced to the United States from Asia and has become a major pest in orchards in the Eastern United States. To date it has not been recorded in South Africa.

The green stink bug — Nezara viridula — is pentatomid bug found around the world. It feeds on a wide range of plants and is a significant pest of legumes. Green stink bugs have been found in orchards and could be damaging local deciduous fruit.

Another pentatomid is antestia — Antestiopsis thunbergii — which is indigenous to South Africa and a notorious pest of coffee and citrus. Antestia bugs have also been caught in the act of damaging pears.

Two other groups that can damage crops are the myrid or capsid bugs, and the lygaeid or seed bugs. They include species that are known to cause dimpling in apples and pears overseas. A large diversity of bugs may be found in orchards, but many are likely to be either beneficial predators or harmless visitors.

Bugs tend to be sporadic pests in orchards. Their piercing mouthparts puncture fruit and induce superficial damage to the flesh. Developing fruit become dimpled and deformed. Older fruit can have dimples and depressed areas. The flesh may be discoloured or have a corky area beneath the dimple. Affected fruit can suffer secondary invasion by spoilage organisms such as bacteria and fungi.

Bug damage can be hard to distinguish from pear stony pit. So what is causing dimpling in South African orchards? Hortgro aims to find out by funding a new project that will start later this year. Read more about it in our article Figuring Out The Cause Of Pitted Pears.

Image: Dimpling and internal damage found in pears from a South African orchard.

Supplied by Hano Maree | Stellenbosch University.