A dark, mouldy coating of ascomycete fungi living on honeydew excreted onto fruit and leaf surfaces by various sap-sucking insects such as aphids, mealy bugs, planthoppers, scale insects and whiteflies. The fungi do not feed on the plant but live on the honeydew.
General comments:
Sooty mould causes unsightly black marks that downgrade fruit in the market. The growth of the fungi is more prolific during wet, humid weather.
Identification:
A dark, dense sooty layer on leaves and fruit that, unlike sooty blotch, is easily rubbed off. The layer can get quite dense and acquire a felt-like appearance. In severe cases, leaves can become heavily covered, turn yellow and may fall off prematurely. Symptoms are often worse in trees alongside dusty roads where dust interferes with natural control of sap-sucking pests by beneficial insects.
Sooty mould on leaves. In this photograph, scale insects are responsible and can be seen feeding on the twigs.Sooty mould on fruit. Sooty mould is usually thicker and more ‘velvety’ than sooty blotch and can be rubbed off the skin easily.
Treatment/prevention:
Find out what insects are producing the honeydew and take action to control them. This does not necessarily mean applying more insecticide sprays; on the contrary, the proliferation of sap-sucking insects is often due to the overuse of insecticides that has killed off the beneficial insects. Provide better ventilation to reduce dampness and humidity by pruning and shaping trees to keep the canopy open.
Sooty blotch is widespread throughout the wet subtropical and coastal regions. It is a superficial blemish but reduces the market value of fruit.
Identification:
It can occur on all plant parts, especially in the shaded parts of the tree. Spores germinate readily on plant parts during warm, rainy weather to produce uneven smoky, sooty superficial blotches on the surface. Unlike sooty mould it does not need honeydew produced by insects to become established and the dark staining of sooty blotch cannot be rubbed off.
Sooty blotch gives green branches a dirty grey appearance. Sooty mould can look similar but is usually darker and thicker.
An uneven black superficial blemish develops on the skin surface. It is hard to rub off. Often the symptoms are streaked.
Treatment/prevention:
The program of copper sprays applied for anthracnose should adequately control sooty blotch, provided good spray coverage is maintained. Provide better ventilation to reduce dampness and humidity by pruning and shaping trees to keep the canopy open.
The soil-borne water mould Phytophthora cinnamomi.
General comments:
Tends to be isolated to a few trees here and there, for example, when irrigation sprinklers are placed too close to the trunk, the trunk has been damaged, or there has been a cyclone. It has occurred in Hass on Mexican race rootstocks in North Queensland following cyclones, where very wet conditions were combined with physical damage to the tree trunk from flying debris.
Identification:
The canker appears as a dark brown water-soaked area on the bark. Healthy bark is normally a grey-brown colour. A white secretion of dried sap is usually evident within the affected area; however, the presence of the white secretion alone does not necessarily indicate trunk canker disease (chronic boron deficiency and injury to the trunk may produce the same symptom). After some time the affected section ‘sinks’ into the trunk.
Active trunk cankers bleed sap that dries to form a white powder.
Lesions, splits and sunken areas on the trunk from older infections.
Treatment/prevention:
Using a sharp knife, pare away all the diseased bark and wood and paint the area with a mixture of a copper fungicide and water-based paint. Alternatively, spray affected trunks with a registered phosphonate fungicide. Protect the trunk from physical damage to prevent the pathogen from gaining entry. Avoid continuous wetness of the trunk from sprinklers or from having mulch material against the trunk.
The fungus Colletotrichum gloesporioides, the same pathogen that causes anthracnose.
General comments
Pepper spot is observed on fruit in the field, unlike anthracnose, which mostly doesn’t develop until fruit is ripening. It is more likely to occur on fruit surfaces that are affected by sunburn, even mild sunburn, and in trees that are stressed (e.g. by drought or Phytophthora root rot). It is a cosmetic problem only affecting the fruit surface, but it downgrades the fruit. Pepper spot is known as ‘speckle’ in South Africa.
Identification
Numerous minute, shiny black, raised spots (0.1 mm to 0.5 mm in diameter) develop on the surface of the fruit, fruit stems and twigs, especially on the warm northern and western sides of trees. Symptoms appear from mid to late summer. Spots are particularly prevalent on Hass fruit. Sometimes ‘tear stain’ marks are formed on the fruit surface.
Tiny black raised spots develop on green twigs and fruit.
Pepper spot consists of hundreds of very small, raised, black and shiny spots on fruit, fruit stalks and leaf stems. At times they merge together to form black patches. Pepper spot is most likely to be found on mildly sunburnt parts of the fruit.
Sunburn predisposes fruit to pepper spot.
Treatment/prevention
Take steps to minimise exposure of fruit to sunburn; this includes keeping phytophthora root rot under control.
The fungicide spray program recommended for anthracnose control helps to control pepper spot.
The disease was identified in Australia in 1992 in several orchards on the Atherton Tableland of North Queensland, but the disease does not appear to have spread from there. Varieties vary in susceptibility, with Fuerte and Sharwil being far more susceptible than Hass and Shepard. It is a significant disease in South Africa where it is known as ‘black spot’. Fruit quality is reduced, becoming unsaleable. Spores are easily spread by wind and rain to initiate new infections.
Identification
The disease symptoms can appear on leaves, stems and fruit.
Leaves: Symptoms initially appear on the underside of leaves as small (1 to 5 mm), angular, light brown to purple spots surrounded by a yellow halo. The angular shape of the spots and their yellow halo distinguishes this disease from others. The same spots eventually become visible on the upper surface of the leaf. As they grow, individual spots can join to form larger lesions. Under humid conditions grey spore bodies may develop on the spots and can be seen with the aid of a hand lens.
Cercospora spots on the upper (left) and lower (right) surfaces of the leaf.
Close-ups of spots on the upper surface. Spots are angular and light brown to purple with a yellow halo.
Fruit: Spots are initially small, raised and black, becoming slightly sunken and cracked over time. Other fungal diseases such as anthracnose can invade the fruit through these cracked lesions.
Raised black spots become slightly sunken and cracked as the disease develops.
Skin sliced off the fruit shows the depth of the lesions.
Stems: Dark brown to black irregular lesions can develop on green twigs and fruit stems causing small fruit to fall off.
Treatment/prevention
Generally well-controlled by the fungicide spray program recommended for anthracnose control.
This disease has become more prevalent and widespread since the early 2000s. To date it has been found in orchards from the Atherton Tableland in North Queensland south to Northern New South Wales. It has a very wide range of host plants and has been found in a range of environments. It can survive for many years in roots and woody debris in the soil. The disease spreads from infected trees via root contact.
Identification
The most obvious symptom is a sudden whole-tree collapse caused by a girdling of the vascular system. Leaves suddenly wilt and turn brown but may remain attached to the tree for many weeks. The whole tree is affected and always dies. In some cases there is a slower deterioration in tree vigour and leaves become pale and drop prior to complete tree death. A brown crusty ‘stocking’ growing up the trunk from the root collar may also be present. Particles of soil and mulch are bound onto this ‘stocking’ at the soil level and the wood underneath is dead. The disease normally spreads sequentially along the row from the first tree that is infected. While Phytophthora cinnamomi only affects the white feeder roots, Phellinus noxius advances up the large roots all the way to the trunk, encrusting the roots with a blackened infection stocking. Brown root rot can be confused with waterlogging and severe frost damage. Verticillium wilt causes similar sudden death in the canopy but this is usually confined to a single branch and unless it is very young, the tree recovers.
Brown root rot causes the leaves of the entire canopy to wilt suddenly and then die within a few days.
The characteristic infection ‘stocking’ moving up the trunk.
The wood has been cut back to reveal a clear demarcation between the dead wood behind the ‘stocking’ on the left-hand side and the yet-to-be-affected trunk area on the right-hand side.
Brown root rot is transmitted along the tree row via root-to-root contact and all infected trees die. In this photo, infection has moved along the row from right to left. The apparently healthy tree on the left is likely to be infected already.
Treatment/prevention
In fields being prepared for avocado planting, remove from above and the below ground all sticks and roots bigger than one centimetre in thickness and allow enough time for smaller sticks to rot away before tree establishment. This disease is thought to be able to survive only in plant material. Where an infected tree has been positively identified, remove the whole tree as well as the tree on either side of it (even if they are not showing symptoms they are probably already infected) together with all wood and roots. This material should be burnt—do not chip and use as mulch. Install root barriers around the infected soil area to prevent roots from healthy trees coming into contact with infested woody material. No treatment for infected trees is currently available and it is difficult to re-establish new trees in the same planting site.
The fungus Calonectria ilicicola (formerly Cylindrocladium parasiticum).
General comments
A recently described root disease in Australia, which can cause severe damage to young trees and often leads to tree death.
Identification
The disease affects young trees before and after planting out. In milder cases the above-ground part of an infected tree may appear healthy but growth will be stunted. Roots show large areas of severe dark brown or black dead tissue, as well as smaller, brown lesions. The disease is favoured by high humidity, wet conditions, shading and high temperatures. The extent of the disease can be made worse by transplant shock and over-irrigation. Dieback in newly planted trees usually starts in the youngest tips and is accompanied by extensive root death. It can lead to large losses of trees in young avocado plantings.
Black root rot can be confused with Phytophthora cinnamomi root rot; however there is no evidence that black root rot kills older, more established trees. Phytophthora cinnamomi usually causes a slow decline with all-over canopy decline, whereas Calonectria ilicicola is more rapid. Feeder root death can also be caused by brown root rot and natural dieback at flowering time.
Dark brown or black feeder roots can be seen in young trees in the nursery or at planting out.
The youngest part of the tree is usually affected first (top) and sometimes the whole tree dies (bottom).
Treatment/prevention
Good nursery hygiene is essential. Try to avoid planting out during periods of wet weather, high humidity and high temperatures. Take care not to over-irrigate.
The bacterium Pectobacterium carotovorum (formerly Erwinia carotovora).
General comments
Bacterial soft rot does not occur often but can be a major problem when the right conditions arise.
Often seen after cyclones or other extended rain events that are accompanied by strong winds.
Physical damage to wet fruit e.g. caused by wind-driven debris or debris thrown up by machinery especially slashers, can lead to soft rot especially on low hanging fruit exposed to soil splash.
Most likely to be seen after harvest in fruit that was picked during rain and has some injury to the skin (e.g. anthracnose, insect damage or mechanical injury).
An infected fruit on the sorting belt can soon infect many more.
Widespread in Shepard fruit following Cyclone Larry (in North Queensland) due to the combination of wind damage to fruit and very wet conditions.
The disease inoculum can be splashed from the soil onto low-hanging fruit.
Identification
Fruit has a putrid smell.
Externally, the fruit initially has a darkened metallic sheen then the skin develops cracks as the disease progresses
Internally the flesh has soft, grey to black water-soaked areas of rot.
Green skin varieties such as Fuerte, Sharwil, Shepard and Reed are most susceptible, particularly when over-mature at harvest.
A metallic sheen is typical of early symptoms of bacterial soft rot.
As the disease advances the skin cracks and the flesh rots, emitting a putrid smell.
Symptoms develop progressively through the flesh and are accompanied by a putrid smell.
Treatment / Prevention
Manage pests effectively.
Use windbreaks to reduce fruit injury from wind rub.
Consider skirting to avoid having fruit close to the ground that are exposed to soil splash
Do not harvest during wet weather.
During wet weather, do not operate machinery in the orchard that could damage fruit.
Harvest fruit carefully to avoid skin injuries
In susceptible varieties consider clip picking rather than snap picking.
Do not use dip tanks to wash fruit – once a diseased fruit has passed through, there is potential for every subsequent fruit to be infected.
Considered a very minor disease with insignificant consequences, does not appear to damage the tree but will reduce photosynthetic capacity of affected leaves
Requires wet, humid, and shaded environments for infection
Under prolonged moist conditions the algae may cover the whole leaf
Identification
The algal spots are whitish to pale yellow and are most common on the upper leaf surfaces. They become reddish brown as they age due to spore development.
Algal spots can range from white to yellow to reddish brown (close-up on right).
Treatment / Prevention
Manage tree canopies to improve light penetration and ventilation.
The disease does not warrant spraying, however copper fungicides applied to control other diseases will reduce algal growth.
Crop cycle calendars have been prepared for each of the main production regions of Australia.
Please click on “Related Resources” above to view all Crop Cycle Calendars.
These calendars are intended to give the grower a guide to the main management practices, such as pest and disease control, due at different times in the seasonal crop cycle. Further detail about each practice can be found in other sections of the Best Practice Resource. Growers can use the calendars as memory joggers for what needs to be undertaken in the orchard at different times of the year. Please be aware that variations occur between seasons and between localities within each region so growers need to exercise judgment in following the timing suggested. Management activities should be linked to events in the crop cycle rather than to the month of the year.
Active trunk cankers bleed sap that dries to form a white powder.
Lesions, splits and sunken areas on the trunk from older infections.
Tiny black raised spots develop on green twigs and fruit.
Pepper spot consists of hundreds of very small, raised, black and shiny spots on fruit, fruit stalks and leaf stems. At times they merge together to form black patches. Pepper spot is most likely to be found on mildly sunburnt parts of the fruit.
Sunburn predisposes fruit to pepper spot.
Close-ups of spots on the upper surface. Spots are angular and light brown to purple with a yellow halo.
Raised black spots become slightly sunken and cracked as the disease develops.
Skin sliced off the fruit shows the depth of the lesions.
Brown root rot causes the leaves of the entire canopy to wilt suddenly and then die within a few days.
The characteristic infection ‘stocking’ moving up the trunk.
The wood has been cut back to reveal a clear demarcation between the dead wood behind the ‘stocking’ on the left-hand side and the yet-to-be-affected trunk area on the right-hand side.
Brown root rot is transmitted along the tree row via root-to-root contact and all infected trees die. In this photo, infection has moved along the row from right to left. The apparently healthy tree on the left is likely to be infected already.
Dark brown or black feeder roots can be seen in young trees in the nursery or at planting out.
The youngest part of the tree is usually affected first (top) and sometimes the whole tree dies (bottom).
