Plant problems

Why Are My Boysenberry Leaves Curling?

Q: Why are my boysenberry leaves curling?

Boysenberry (a complex hybrid involving loganberry, blackberry, and raspberry) leaves curl most commonly in the UK because of aphid infestation, raspberry leaf and bud mite (Phyllocoptes gracilis), or cane disease. The raspberry aphid (Amphorophora idaei) colonises the growing tips of boysenberry canes in spring, causing the young leaves to curl and cup downward; heavily infested shoot tips become stunted and distorted. Raspberry leaf and bud mite (Phyllocoptes gracilis) is a microscopic eriophyid mite invisible to the naked eye that causes a distinctive irregular, crinkled, fan-like distortion of the youngest leaves at the shoot tips, with pale yellowish mottling along the veins; the distortion is different from aphid curl (tightly cupped) and more diffuse and puckered in character. Cane blight (Paraconiothyrium fuckelii) and spur blight (Didymella applanata) can cause the tips of current-season canes to wilt and curl above dark lesions, particularly in wet summers.

Q: What is the history of the boysenberry?

Boysenberry was developed by Rudolph Boysen, a California horticulturist, in the 1920s. Boysen crossed loganberry, blackberry, and raspberry to produce a new hybrid with large, dark purple, winey-flavoured berries. He grew the plants on his farm but abandoned them; the plants were later discovered in a neglected state and identified by the American horticulturist Walter Knott, who propagated them with Boysen's agreement and began selling the fruit at his roadside stand in Anaheim, California, around 1935. Knott named the berry after Boysen and built a successful business around it; Knott's Berry Farm, which began as a berry-selling operation, eventually became one of the first theme parks in the United States. Boysenberry was introduced to New Zealand and Australia, where it became a popular commercial crop; in the UK it is grown primarily as a garden fruit. It produces long, vigorous, arching canes that need strong support and can reach 3 metres or more; it is fully hardy in most UK gardens and very productive in good summers.

Q: How do I grow boysenberry in the UK?

Boysenberry grows well in UK gardens as a productive, vigorous cane fruit. It is fully frost-hardy and requires the same growing conditions as loganberry and tayberry: a sunny, sheltered position with reasonably fertile, well-drained soil; support wires or a trellis system at 30, 60, 90, and 120 cm; and space between plants of about 2.5 to 3 metres to accommodate the long, arching canes. Boysenberry bears fruit on the previous year's canes; after the canes have fruited, cut them out at ground level and tie in the new season's canes in their place. The large, deep purple berries ripen in late July and August; they have a rich, winey, sweet-tart flavour and are excellent fresh (sweeter than loganberry), as well as in jam, pies, and juice. The canes are thorny; thornless boysenberry varieties are available and are much easier to manage in a small garden. Boysenberry is propagated by tip layering (burying the tip of a long cane in the soil in late summer; it roots readily) or by division of established clumps.

Q: What does boysenberry taste like?

Boysenberry has a large, deep purple to dark maroon berry with a rich, complex, winey flavour that combines the sweetness of blackberry with the sharper, more fragrant notes of raspberry and a hint of the tartness of loganberry. The berries are significantly sweeter than loganberry and less sharp, making them more pleasant to eat fresh; they have a soft, juicy texture and a deep, jammy quality that intensifies on cooking. Boysenberry jam has a distinctive deep wine-purple colour and a rich, complex flavour considered by many to be superior to single-species berry jams. The berries are fragile and must be handled carefully; they do not store well and should be used within a day or two of picking or preserved immediately. Boysenberry is a popular flavour for yoghurt, ice cream, and fruit drinks in New Zealand and Australia, where commercial production is well established. In the UK the berries are less widely known but equally delicious; a UK-grown boysenberry in August, eaten fresh with clotted cream, is considered one of the finest soft fruits available.

Boysenberry is a complex hybrid of loganberry, blackberry, and raspberry, developed by Rudolph Boysen in California in the 1920s and popularised by Walter Knott, whose berry stand eventually grew into Knott's Berry Farm. It produces large, deep purple berries with a rich, winey flavour and is fully hardy across UK gardens. As a close relative of raspberry and loganberry, it shares the same spring aphid problem and susceptibility to the invisible leaf mite that causes distinctive leaf distortion on cane fruits.

Aphids

The raspberry aphid (Amphorophora idaei) is the most common cause of leaf curl on boysenberry canes in spring, colonising the growing tips from April onward and causing the young leaves to curl and cup tightly downward. Heavily infested shoot tips become stunted and distorted, with a dense cluster of small green aphids visible on the undersides of the curled leaves. The raspberry aphid is also a vector of raspberry mosaic virus complex, which can cause persistent poor cropping. A secondary aphid species, the blackberry aphid (Amphorophora rubi), can also colonise boysenberry but is less common than the raspberry aphid.

What to do

Check growing tips weekly from April. Pinch off and destroy the most heavily infested shoot tips. Apply insecticidal soap or neem oil to the shoot tips and leaf undersides, repeating every 7 days. Encourage ladybirds, lacewings, and parasitoid wasps. Avoid high-nitrogen feeds in spring, which produce the soft, sappy growth aphids prefer. Replace plants that show persistent mottling or distortion after aphid control; these may have acquired viral infection.

Raspberry leaf and bud mite

Raspberry leaf and bud mite (Phyllocoptes gracilis) affects boysenberry as it does loganberry, tayberry, and raspberry. The microscopic mites are invisible without magnification and cause an irregular, puckered, crinkled distortion of the youngest leaves at the shoot tips, with pale yellowish mottling along the veins. The distortion has a characteristic diffuse, fan-like quality that distinguishes it from aphid curl (which produces tightly cupped, closed leaves). Severely infested plants show reduced cropping. No chemical control is available; the response is removal and replacement with certified mite-free stock.

What to do

Remove and destroy the most affected shoot tips. If most of the plant shows the characteristic crinkled distortion, remove and replace with certified mite-free stock. Do not take cuttings or divisions from affected plants. Buying from certified suppliers is the most effective prevention.

Cane disease

Boysenberry can be affected by cane blight (Paraconiothyrium fuckelii) and spur blight (Didymella applanata), both of which can cause wilting and curling of the tips of current-season canes. Cane blight causes sudden wilting of a whole cane tip above a dark, cracked lesion near the base of the current-season cane; spur blight causes dark lesions at the nodes that girdle the lateral shoots and cause them to die and curl. Both diseases are spread by splash-dispersed spores in wet weather and overwinter on infected canes. Boysenberry appears generally less susceptible to cane spot (Elsinoe veneta) than loganberry.

What to do

Remove and destroy infected canes at ground level after harvest. Thin to 8 to 10 canes per plant to improve air circulation. Avoid overhead watering. Apply a copper-based fungicide spray in early spring if cane disease was a problem the previous year. Never compost infected material.

Grey mould

Grey mould (Botrytis cinerea) can colonise the shoot tips and flowers of boysenberry in cool, damp UK conditions, causing the tip leaves to brown, collapse, and curl over. The grey, fuzzy spore masses of Botrytis are visible on badly affected tissue. Grey mould is most problematic on the ripening berries (which can rot quickly in a wet summer) but also affects the growing tips in spring if they have been damaged by cold, drought, or pest feeding.

What to do

Improve air circulation by thinning canes. Remove and destroy all affected tissue. Avoid overhead watering. Apply a sulphur-based fungicide preventively in wet springs. Ensure canes are well tied to a support system so they are not touching the ground.

Frequently asked questions

Why are my boysenberry leaves curling?

Boysenberry leaves curl most commonly in the UK because of aphid infestation, raspberry leaf and bud mite (Phyllocoptes gracilis), or cane disease. The raspberry aphid colonises growing tips in spring, causing leaves to curl and cup tightly downward. Leaf and bud mite causes a diffuse, puckered, crinkled distortion of the youngest leaves at the shoot tips, invisible without magnification. Cane blight and spur blight cause tips to wilt and curl above dark lesions on the current-season canes.

What is the history of the boysenberry?

Boysenberry was developed by Rudolph Boysen in California in the 1920s by crossing loganberry, blackberry, and raspberry. The plants were discovered abandoned and then popularised by Walter Knott from around 1935; his berry-selling business eventually became Knott's Berry Farm. Boysenberry is a popular commercial crop in New Zealand and Australia and is grown in UK gardens as a vigorous, productive cane fruit with large, rich, winey berries.

How do I grow boysenberry in the UK?

Boysenberry is fully frost-hardy and grows in any reasonably fertile, well-drained UK soil in a sunny position. Support on horizontal wires at 30, 60, 90, and 120 cm, spacing plants 2.5 to 3 metres apart. After fruiting, cut out the fruited canes at ground level and tie in the new canes. Thornless varieties are much easier to manage. The large, deep purple berries ripen in late July and August with a rich, winey, sweet-tart flavour.

What does boysenberry taste like?

Boysenberry has a rich, complex, winey flavour combining the sweetness of blackberry with the fragrant notes of raspberry and a hint of loganberry tartness. The large, deep purple berries are sweeter than loganberry and pleasant to eat fresh; they have a soft, juicy, jammy quality that intensifies on cooking. Boysenberry jam has a distinctive deep wine-purple colour. The berries are fragile and should be used within a day or two of picking. A popular flavour for yoghurt and ice cream in New Zealand and Australia.