Growing Avocados-Fungal diseases of fruit

Page last updated: Friday, 29 May 2020 - 9:31am

Avocado fruits are affected by two diseases: stem end rots and body rots. Body rots have a few causes but are predominantly due to anthracnose. Stem end rot is characterised by a dark brown to black rot that starts at the stem end of the fruit and proceeds downwards. Management of both diseases is through a holistic approach that combines chemical control and orchard practices. 

Avocado trees are affected by a few serious diseases. The most serious of these are phytophthora root rot, followed by stem end rots and anthracnose of the avocado fruit. Body rots have a few causes but are predominantly due to anthracnose. Rots are usually addressed through postharvest management. However, the holistic management of these rots begins in the orchard. Research from many years has resulted in the current approaches to managing stem end rot and anthracnose. One important component of management is understanding the pest itself. This article will review the current understanding of stem end rot and anthracnose. From this understanding a comprehensive orchard level management of these diseases can then be derived.

Stem end rot is characterised by a dark brown to black rot that starts at the stem end of the fruit and proceeds downwards. Stem end rot is caused by species of Colletotrichum as well as Dothiorella, and other species such as Phomopsis (syn. Diaporthe) and Botryosphaeria. Anthracnose is responsible for both leaf and fruit lesions. Internationally, it is principally caused by Colletotrichum gleosporioides (Nelson, 2008). In Australia C. acutatum is also responsible for anthracnose but less so than C. gleosporioides. In south west WA a cold tolerant strain of C. acutatum called C. fioriniae is also present (Pers comm. Simon Newett).

These fungi have a wide host range, having been found on most woody horticultural crops and within native bushland. They are present on the leaves, branches, twigs, the fruit and the peduncle. During rain events the inoculum or spores for anthracnose or stem end rot are washed onto the fruit. The specific mechanism to which infection occurs requires further study. For Stem end rot only the pathway of infection is an endophytic one, whereby the fungus is already growing on the fruit before harvest. When the fruit stem is damaged, either naturally or via the picking process, the fungus becomes necrotic and grows down the vascular bundles of the fruit, thus initiating stem end rot.

The mechanism of infection by anthracnose is initially similar to stem end rot but how the fruit is neurotically infected is different. As per before the inoculum and spores are already on the tree. Then, rain showers spread the anthracnose, and the warm weather facilitates its growth on the surface of the fruit. Initial growth infects the fruit but stops after penetrating the cuticle 9essentially the outer skin of the fruit). The fungus is kept in an inhibited state by antifungal compounds, one of which is called a diene. Or in other words, the infection has occurred at some stage during the growing season but remains latent within the fruit. As the fruit ripens the diene content in the fruit is reduced. Unfortunately the decrease in dienes allow the anthracnose to resume growth and damage the fruit. (Willingham et al., 1997).

Management of both of these fungal diseases begins with removing dead material as is practical from the canopy and orchard floor, hence removing inoculum from the orchard. Removing the dead material will remove the primary source of inoculum from the tree. Keeping the canopy open is also beneficial as it creates gaps between branches, which can reduce the transfer of inoculum throughout the tree. Open spaces in the tree also facilitate air movement, which allows excess water to dry off after rain more quickly than in a dense canopy. Drier conditions are not as ideal for fungal growth. Picking method has been shown to be important in New Zealand with snip picking instead of snap picking being shown to reduce stem end rot. However, this is in the extremely wet conditions of New Zealand and is probably unlikely to be useful in the south west. Snap picking is quicker and is the best way of harvesting Hass in our conditions. Other varieties such as Lamb Hass need to be snip picked as they are more susceptible to stem end rot. With regards to picking fruit more consideration needs to be given to the wetness of the fruit. Wet fruit are a more easily mechanically abraded (Duvenhage, 1993). As injuries to the fruit allow the fungus in it is best to avoid damaging the fruit. Thus, picking in wet weather is best avoided.

Management of both diseases is strongly implicated with the health of the tree and mineral nutrient status. Research in New Zealand has shown that as calcium levels increased in the fruit the incidence of fruit rots decreased.(Everett, 2014). With regards to anthracnose an increase in plant nitrogen status caused an increase in anthracnose severity while lower nitrogen reduced anthracnose severity (Willingham et al., 1997). Caution needs to be advised with interpreting nitrogen in terms of anthracnose susceptibility. Those treatments in this study with high nitrogen may have had absurd nitrogen levels causing softer wood which is much more susceptible to infect by anthracnose. There is also a rootstock effect. Guatemalan rootstocks like Velvick accumulate more calcium and magnesium than Mexican rootstocks. However it is not clear if this makes an impact on fruit quality. Velvick does have more antifungal dienes in its leaves than the Mexican rootstock Duke 6 so it has less anthracnose as the tree canopy is less favourable to fungal infection.

A few other factors were analysed on some of this research in New Zealand. Thicker mulches reduced stem end rot, as did a warmer climate and having less rainfall before harvest. A canopy index was developed as an index to quantitatively reflect how thick the canopy was and how much dead wood there was. There was a small relationship between canopy index and fruit diseases with a larger index resulting in more disease. The relationship with the canopy index goes back to a point made earlier in this article; lots of dead wood in canopies and dense canopies favour fungus growth and spread. Ultimately the largest relationship was between Ca + Mg: K ratio and body rots. A higher ratio, that is more calcium and magnesium and less potassium, reduced fruit rots (Everett et al., 2007).

The main means of managing fruit diseases is the use of fungicide, chiefly copper based fungicide. Copper fungicide use has been shown to reduce fruit rot incidence from 100% to 6.8%. Interestingly, copper fungicide initially does not reduce fruit fungi infections Instead it increases them, likely due to the removal of beneficial fungi that are antagonistic to the disease causing fungal species present. More than three sprays worsened fruit rots while more than eight reduced them. Six to seven also increased fruit rots (Everett et al., 2007). Therefore copper sprays are an important part of effectively reducing fruit rots but do need to be applied regularly and in a high enough number to be effective. On the Avocados Australia best practice resource (free to become a member) there is a great description of a good copper fungicide program (Table 1). While it is made for the warmer climate of Queensland, the fungal species listed are able to grow in the cooler temperatures of Western Australia. Therefore the fungal program for Queensland is appropriate for South West Western Australia.

In addition to copper azoxystrobin can be used. Unlike copper it is not a protectant and will act as a curative to kill off existing infections. Azoxystrobin has one site of action and consequently has limitations on its use. It cannot be used as either the first or last spray in the season, can only be used three times, cannot be used consecutively and can only make up 1/3 of all sprays done in the season. These limitations are to reduce the chance of resistance developing. The interval between an azoxystrobin and copper spray can be treated the same as for a copper only spray regime; 14-28 days depending on the weather.

Table 1. Recommended copper spray interval. This spray program comes from the Avocados Australia Best practice resource. Do not mix copper with phosphorus acid sprays, due to the potential for phytotoxicity. In the south west the spray interval can continue through the winter as the local anthracnose species grows at low temperatures. 

Type of weather

Copper spray interval

Warm weather

Every 28 days

Some rain

Every 21 days

Heavy rain

Every 14 days

 

Future directions for fruit management

Alternatives to chemical control are always desirable due to both social influences and due to the fungal species themselves developing resistance to chemicals. Bacillus subtilus has been trialled in South Africa to determine if it can control fruit diseases in the late 1980s. In Australia similar research has been published in 1995, which showed a species of Bacillus inhibited anthracnose (Dann et al., 2013). As of the time of publication there is a registered product on the APVMA which uses a species of Bacillus and is registered for avocado to control anthracnose. There was also initiative in Australia which attempted to manipulate the natural antifungal compounds in the avocado fruit and tree to manage fungal diseases. The research I described earlier that mentioned dienes was the output of that research initiative. While dienes varied in different rootstocks there was no other means of control outside of rootstock choice that made a difference. Silicon has also been trialled on avocado trees. When soluble silicon was injected into avocado trees anthracnose was reduced (Anderson et al., 2005). While promising more research is required.

Avocado fruits rots are caused by stem end rot and anthracnose. The causative fungi of these rots live and grow in tree canopies and preference canopies that are dense with lots of dead wood. Trees with less calcium are more susceptible to these fungi. Control of these fungi needs to be done as a whole with management of the canopy, removal of dead wood, not picking during wet weather, managing calcium nutrition well, using the correct rootstock, and using copper and azoxystrobin appropriately. New options of control have been attempted in the past and presumably more will be attempted in the future.

References

Anderson, J. M. et al. (2005) ‘New strategies for the integrated control of avocado fruit diseases’, New Zealand and Australia Avocado Growers Conference 2005, (3), pp. 1–6.

Dann, E. et al. (2013) ‘Foliar, fruit and soilborne diseases’, The Avocado: Botany, Production and Uses, pp. 380–422.

Duvenhage, J. A. (1993) ‘South African Avocado Growers’ Association Year book’, 16, pp. 77–79. Available at: http://www.avocadosource.com/Journals/SAAGA/SAAGA_1993/saaga_1993_pg_77-....

Everett, K. R. et al. (2007) ‘Calcium , fungicide sprays and canopy density influence postharvest rots of avocado’, pp. 22–31.

Everett, K. R. (2014) ‘AVOCADO FRUIT ROTS : A REVIEW OF INDUSTRY FUNDED RESEARCH’, (January 2002).

Nelson, S. (2008) ‘Anthracnose of Avocado’.

Willingham, S. et al. (1997) ‘CONTROLLING ANTHRACNOSE IN AVOCADO BY ENHANCING NATURAL FRUIT RESISTANCE : THE ROLE OF ROOTSTOCKS AND’.

Author

Declan McCauley