Why is my olive fruit so small? Do I have Shotberries?

Contacted by a grower who has this problem with their olive fruit this season: 

We experienced a problem with our olives.  We had a leaf and soil test done and is within the parameters. The trees flowered extremely well; however, in most of the trees although the flower did set the fruit does not grow. Perhaps one out of every 1000 fruit set properly.  The small fruit does not drop off but has increased in size to about 4mm.  This is getting progressively worse and we are desperate to find an answer.  Can you help?

What are Olive Shotberries?

Shotberries are small unviable olive fruits that will grow to a maximum of about 4mm.  There is usually an abundance present per tree.

Other names: ‘Chicken Fruit’ or ‘Parthenocarpic fruit’

What are the symptoms?

Lots of small unviable fruit which mature earlier than normal olive fruit.  Normal fruit on the tree is in the minority.

How does this happen?

Shotberries are thought to be a pollination issue but are usually caused by adverse weather conditions during flowering/pollination process.  Usually, during a hot/dry spell or on the other hand a cool/wet spell can also have this effect.  At the time when conditions are favourable to being hot/dry or cold/wet  i.e. when the pollen has not reached the flower ovule (therefore not able to set properly) because the pollen is not strong enough.  A lack of boron may have contributed.  This situation may arise in a 3 – 5 year period.  If this circumstance arises a very heavy fruit set normally follows and can even bring on alternate bearing issues.

Varieties like Manzanillo and Sevillano are more prone to shotberries.

A study of pollination by Hartmann suggests tree vigour may also be related to the production of shotberries.  Trees with greater vigour tend to have a greater incidence of shotberries.

Olive Flowers are very sensitive to adverse weather conditions.

What comes next?

If the crop is greatly reduced because of shotberries, it sets the grove up for extreme alternate bearing followed by a light crop.  A very heavy crop is likely the next season.

What can be done to prevent this?

To reduce the impact of shotberries consider exploring more cross-pollination options in your grove and reduce alternate bearing.  Are you growing only one cultivar?  Ask yourself if there was there any adverse weather conditions during flower development & bloom?  To mitigate future cropping it would be advisable to develop a contingency plan with a Specialist Olive Consultant.

See more about getting advice from a Specialist Olive Consultant

Sources:

Ovule development in normal and parthenocarpic olive fruits

HF Rapoport, L Rallo – International Symposium on Olive Growing 286, 1989 – actahort.org

Olive flower and fruit population dynamics

GC Martin – International Symposium on Olive Growing 286, 1989 – actahort.org

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