A number of serious diseases can reduce yields and kill passionfruit 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.
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 results 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.
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 suddently 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.
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 leasions causing premature drop and fruit decay.
Bitter Rot
The fungus Glomerella cingulata causes this disease and can be responsible for significant fruit loss, especially in hot, humid conditions in the summer months. Infection is facilitated by any damage to the skin of the fruit.
Symptoms
Dark, soft lesions rapidly develop on infected fruit that can colonise whole fruit. Infected fruit will fall to the ground.
Control
The routine protective fungicide programme based on copper sprays will afford protection but under stringent infection conditions may need to be repeated at short intervals in the height of summer.
Picking up and removing infected fruit is a good hygienic practice.
Sclerotinia
Sclerotinia sclerotiorum is the causal pathogen of this disease.
Symptoms
It can affect stems where lesions can enlarge to cause a girdling and collapse of the shoots 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.
It can also infect fruit with infections rapidly becoming pale brown lesions that can develop over a whole fruit. With advanced infections a mass of fluffy white fungal growth is produced in which the black sclerotes can be seen. Infected fruit falls to the ground, and the sclerotes will carry the infection potential over to future seasons.
Infection occurs mainly in the November-December period when vines are wet for 16-24 hours and temperatures are in the 15-200C range.
Control
Where Sclerotinia has been a problem application of a suitable fungicide after pruning when the main structure of the plant can be covered is worthwhile.
Orchard hygiene is also important. Infected fruit should be picked up and removed from the orchard, and infected shoots cut out below the lesion and destroyed. This will ensure that the sclerotes are removed and minimise the carryover of infection within the orchard.
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 Fusariumavenaceum, Gibberella baccata, and Gibberella saubinetii.
Symptoms
The condition usually occurs close to ground level, centred on wounds caused by frost, growth cracks, mechanical damage, fertilizer or herbicide burn, or by pest injury such as that caused by slugs or snails. Infections often progress to girdle the stem at or near ground level.
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, fertilizer or herbicides.
It is also good practice to keep the base of the plant clear of grass and weeds, which favour fungal growth and harbour slugs and snails. Where collars are placed around stems to protect against herbicide damage, slug pellets placed inside the collar will help control these pests.
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 the 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.
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
Intected 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, the passionfruit woodiness virus (a member of the potato virus Y group) or alfalfa mosaic virus, or a combination of these or possibly other viruses as well.
Symptoms
On leaves it causes yellow spots, flecks or mottling, and 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 possibly also by pruning tools. Once vines become infected there is no known control. Obviously infected plants should be rogued out and destroyed. Virus symptoms are minimised by promoting vigorous vine growth and where necessary aphids should be rigorously controlled.
In older blocks or where some vines may be infected, disinfecting pruning tools between vines or parts of a block could be worthwhile. This can be achieved by dipping pruning tools in a 1% solution of hypochlorite (bleach) for a few seconds, neutralise by a dip in vinegar and protect tools with a spray of CRC or Valvoline.