New strategies for the control of avocado fruit diseases (AV01004)

This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

Loss of avocado fruit in the marketplace due to anthracnose and stem-end rots was still a major concern for both growers and consumers. The ability of the rot causing fungi, to remain in a dormant or latent phase until after the fruit commenced ripening made disease control very difficult. In a previous project (AV97001), significant advances were made to control rots with the world first discovery that rootstock race affected the severity and incidence of fruit rots. This project aimed to advance these rootstock studies as well as investigate new products available for disease control, assess harvesting methods and evaluate other strategies such as cross-protection using non-pathogenic strains of the fungus (Colletotrichum gloeosporioides) that caused anthracnose rots. This project also included a PhD study into the preharvest disease, pepper spot. This research was also based on a discovery made in the previous project (AV97001) that pepper spot and anthracnose were caused by the same fungus, Colletotrichum gloeosporioides. The pepper spot investigations aimed to answer the questions of how and why the same fungus was able to cause two different disease symptoms and how it could be controlled.

A series of field experiments were conducted to broaden the rootstock and nutrition research. One set of field trials examined if the form of nitrogen fertiliser (ammonia vs nitrate) affected disease in ‘Hass’ fruit grown on ‘Velvick’ or ‘Duke 6’ rootstocks. The West Indian ‘Velvick’ rootstock was found to have a less vegetative canopy, higher crop load, better balance of mineral accumulation and less anthracnose than the Mexican ‘Duke 6’ rootstock. However, this rootstock effect was lost in the second season, possibly due to very high disease incidences. The form of nitrogen fertiliser did not affect anthracnose rots. However, withholding nitrogen fertiliser resulted in trees with less vegetative canopies, a better balance of nutrients, lower fruit skin pH levels and a tendency to have fewer anthracnose rots.

Another series of rootstock experiments compared a broader range of rootstocks for disease susceptibility. The Guatemalan (‘A8’, ‘A10’, ‘Nabal’) rootstocks were also found to be superior for disease control compared with the Mexican (‘Parida 1’) rootstock. These rootstock effects however, were not apparent in the last two seasons, possibly due to heavier crop loads on ‘Parida 1’. In both rootstock studies, strong relationships between disease levels and mineral nutrient concentrations (in particular nitrogen and calcium) were evident. This suggested the balance of nutrients in the tree had an important predictable impact on fruit rots.

No new products were discovered for superior disease control but harvesting method recommendations were devised. Cross-protection studies require further research but still provided potential for new disease control strategies in the future.

The pepper spot studies discovered the fungus causing pepper spot and anthracnose belonged to the one population but this population was highly variable. However, the same fungus causing anthracnose on mango was less aggressive on avocado and forms its own distinct population. This meant the cross-infectivity potential of the fungus from mango to avocado was low in mixed orchards. The severity of pepper spot in orchards could be minimised by reducing tree stress and using Guatemalan or West Indian instead of Mexican rootstocks. It appeared the fungus infected the fruit in a similar manner to cause both pepper spot and anthracnose but different symptoms were observed primarily due to differences in the fruits response. A better understanding of this differing fruit response may have revealed new disease control strategies for the future.