A number of serious diseases can reduce yields and kill vines if not controlled.

GREASE SPOT

This is caused by the bacterium Pseudomonas passiflorae and is one of the most serious diseases of passionfruit in New Zealand. It infects leaves, stems, and fruit, leading to severe crop losses and even death of vines.

Grease spot showing dark oily spots.

Symptoms:
On leaves, it causes irregular olive-green to brown lesions, often surrounded by a light-yellow halo. If unchecked severe defoliation can result.

On the stems of young growth the first signs of infection are small slightly sunken, dark-green, water-soaked spots. These develop into light-brown, markedly depressed areas.

On older wood, symptoms range from small, slightly sunken, smooth, dark-green circular spots, to large, dark-brown, cracked lesions, which may completely girdle shoots and kill vines.

Early signs of infection on the fruit are small, dark-green, oily spots. These develop into roughly circular, greasy, or water-soaked patches. Premature fruit drop and fruit decay result.

Grease spot is said to be most active in autumn and winter, between March and August. However, a condition known as hard grease spot, also caused by Pseudomonas passiflorae, has become prevalent on passionfruit and is active in summer. The symptoms are similar to ordinary grease spot except that the fruit infections dry out and cause a hard brown patch on the skin, instead of leading to decay. This result in a downgrading of fruit and loss of income.

BACTERIAL BLAST

Blast is caused by a bacterium, Pseudomnas syringae, which is a relative of the grease spot pathogen.

Symptoms and control:
The symptoms of the two diseases are similar, and control measures are the same. Where good control of grease spot is obtained, blast should not be a problem.

BROWN SPOT

This is a serious fungous disease, caused by Alternaria passiflorae, which affects leaves, stems, and fruit. It occurs mainly in spring and early summer.

Brown spot with dark, sunken, circular lesions

Symptoms:
On leaves, small brown spots appear first. These enlarge, develop a lighter-coloured central area, and become irregular or angular in shape.

On stems, elongated dark-brown lesions appear, usually near leaf axils or where stems have rubbed against the supporting wire. Infection spreads from these points and whenever the stem becomes completely girdled the shoot suddenly wilts and fruits collapse.

On fruit, spots first appear as pinpricks, which enlarge into sunken circular lesions with brownish centres. Eventually the rind round the diseased area becomes wrinkled and the fruits shrivel and drop.

SEPTORIA BLOTCH

Caused by the fungus Septoria passiflorae, this disease attacks leaves, stems, and fruit. Even a light infection results in defoliation and premature fall and loss of fruit. The disease is more common during summer and autumn.

Septoria blotch with extensive superficial lesions

Symptoms:
On leaves, tiny, superficial, irregular, light-brown spots appear, quickly followed by severe defoliation as infection spreads.

On stems, spots similar to those on leaves appear. They become deeply sunken but remain minute.

On fruit, the infection initially appears as small spots, similar to those on the leaves and stems. The spots develop into extensive superficial lesions causing premature drop and fruit decay.

BITTER ROT

The fungus Glomerella cingulata causes this disease, which attacks passionfruit to only a limited extent. It is a pathogen which gets in where there is a weakness in the fruit such as a wound, and sometimes causes trouble after frosts or hailstorms, the fungal lesion originating at points of damage.

SCLEROTINIA

Caused by Sclerotinia sclerotiorum, this is a minor disease of passionfruit.

Symptoms:
It can affect stems, where lesions can enlarge to cause a girdling and collapse of the shoot above the lesion. The hard dark sclerotes, which are a means of carrying the fungus over from one season to the next, can often be seen in infected shoots.

Control:
Where sclerotinia is a major problem application of a suitable fungicide immediately after pruning, when the main structure of the plant can be covered, would be worthwhile.

CROWN CANKER

This is a lethal condition of passionfruit, causing sudden wilting, leaf and fruit drop, and death. It has been a major factor in limiting the commercial production of passionfruit in New Zealand.

A number of fusarium species have been isolated from plants suffering from crown canker, the most prevalent of which is Fusarium redolens. Other fungi known to be involved include Fusarium avenaceum, Gibberella baccata, and Gibberella saubinetii.

Severe crown canker at base of plants.

Symptoms:
The condition usually occurs close to ground level, centred on wounds caused by frost, growth cracks, mechanical damage, fertiliser or herbicide burn, or by pest injury such as that caused by slugs or snails.

Control:
Unfortunately there is no known control for crown canker, so all efforts to reduce the risk of infection are worthwhile. These would include selection of the best sites to minimise frost and weather damage, regular frost protection with bracken fern or similar material each winter, and avoiding injury to the base of the plant by implements, fertiliser, or herbicides.

It is also a good practice to keep the base of the plant clear of grass and weeds, which flavour fungal growth and harbour slugs and snails.

Plants suffering from crown canker should be carefully removed and destroyed by burning.

PHYTOPHTHORA ROOT ROT

Phytophthora is known to affect passionfruit. In New Zealand two species are mainly involved, Phytophthora cinnamomi is prevalent in summer and autumn, and Phytophthora megasperma is prevalent in spring.

Both fungi can cause the death of vines, but it is thought that the stress and damage they often cause open the way for invasion by fusarium and death of the plant from crown canker.

Sudden wilting and collapse of a passionvine owing to crown canker.

Control:
Improving drainage is one way to reduce the risk of phytophthora infection.

THIELAVIOPSIS ROOT ROT

This is another fungous disease which can affect passionfruit growing on heavy soils. It is caused by Thielaviopsis basicola.

Symptoms:
Infected plants are unthrifty, with poor-coloured foliage. The roots show signs of decay and are often blackened.

Control:
With mild infections sometimes a severe pruning to balance the top growth with the loss of roots can keep the plants going, but severely infected plants should be removed and replaced.

WOODINESS

Woodiness may be caused by the cucumber mosaic virus or by the passionfruit woodiness virus (a member of the potato virus Y group), or by both viruses in combination, or possibly even other viruses as well.

Woodliness virus symptoms on fruit (left) and symptoms on leaf (right)

Symptoms:
On leaves it causes yellow spots, flecks, or mottling, and in foliage there is crinkling or distortion. It also shows as shortened internodes on the stems, bunching of foliage, and stunted growth. Symptoms are most apparent during late autumn, winter, and early spring.

On fruit it causes thick, hard, distorted woody rinds, often with characteristic scabs and cracks. Pulp yields are much reduced.

Control:
Plant only virus-free plants and remove and replace severely infected vines. The disease is transmitted by aphids and spreads quickly. Once vines become infected there is no known control. The effects are minimised, however, by promoting vigorous vine growth.

DISEASE MANAGEMENT

Control of grease spot, brown spot, septoria blotch, and Glomerella requires a combination of good management, good orchard hygiene, and a suitable spray programme.

Prune out and burn infected material and keep the vines as open as possible to allow thorough application of sprays.

Regular spraying throughout most of the year is essential. During humid weather from November to February, when the vines are growing rapidly, reduce the intervals between spray applications to 2 or 3 weeks to ensure that new growth is adequately protected.

Control will only be as good as the coverage obtained during spraying. It is good practice to check for spray coverage inside dense areas of the canopy.

Where spray coverage is inadequate one or more of the following aspects may need adjustment-tractor speed, pump pressure, nozzle size, or nozzle arrangements.