THE AVOCADO IRRIGATION CONUNDRUM Simon Newett Dept of Agriculture, Fisheries and Forestry, Queensland

Origin of avocados - Central American highlands Central American highlands (e.g. Michoacan State, Mexico) Deep, well drained volcanic soils Soil surface mulch of leaf litter Well structured soils, high in organic matter Benign climate regular rainfall (about 1500mm/yr) absence of temperature extremes Avocado is still a ‘young’ crop: 1911: First release of a selected, propagated variety 1940s: First attempt at rootstock selection

The conundrum . . . . . Avocados have a high water requirement and are sensitive to water stress but they are … inefficient at extracting moisture from the soil sensitive to over watering

Moisture stress = excessive fruit drop

Water stress = ringneck (incomplete fruit abscission)

Water stress = death of seed coat = small fruit

Water stress in first 11 weeks after fruitset = poor shelf life due to low calcium levels

Water stress = poor quality fruit Vascular browning

inefficient at extracting moisture because they don’t have root hairs have a very shallow feeder root system (90% are in the top 15cm of soil) can extract little water below -20 kPa

No root hairs

Shallow feeder root system

Shallow feeder root system

Shallow feeder root system

sensitive to over watering because … feeder roots have a high oxygen requirement (trees die after 48 hours inundation) and … roots are susceptible to Phytophthora root rot (a “new encounter” disease for avocado)

Drowned avocado tree

Waterlogging = fruit fall & uneven ripening

Healthy vs. Phytophthora infected roots

Healthy vs. root rot infected tree

In appreciation of these facts … “A more focussed and responsive approach to moisture monitoring and irrigation is required. This is emerging as a key component in achieving high yields of good quality avocados in Australia.”

The solution Plant only on well drained soil, ensure orchard drainage is near perfect Install an irrigation system that delivers water and fertiliser evenly to every tree and does not leak when turned off Install an accurate & reliable soil moisture monitoring system plus a back up Follow weather forecasts to be prepared for peak demand Monitor soil moisture several times a day in warmer months

The solution (continued) Use the concept of Readily Available Water (RAW) and aim to keep soil moisture between the Full (-8kPa) and Refill (-20kPa) points Respond immediately to irrigation needs, but … … don’t exceed Full Point Irrigate several times a day if necessary Use mulch to create a more hospitable root environment, reduce evaporation, increase soil organic matter and help combat root rot Use overhead misters for cooling during heat waves

Soil moisture monitoring Full point Refill point

The concept of ‘Readily Available Water’ (RAW)

The concept of ‘Readily Available Water’ (RAW) RAW is mm of water in the soil readily available to the plants. For avocados = Full Point (-8kPa) - Refill Point (-20kPa) Soil texture Soil water extractable between -8 and -20kPa (mm/cm) (Mallee soil, 0.5% OM) Sand 0.33 Sandy loam 0.46 Sandy clay loam 0.39 Clay loam 0.30

The concept of ‘Readily Available Water’ (RAW) RAW is mm of water in the soil readily available to the plants. For avocados = Full Point (-8kPa) - Refill Point (-20kPa) Example: Sandy loam Main root zone 15cm 15 x 0.46 = only 7mm Literature suggests that RAW can be increased by 3 to 4mm for every 1% increase in soil organic matter. Soil texture Soil water extractable between -8 and -20kPa (mm/cm) (Mallee soil, 0.5% OM) Sand 0.33 Sandy loam 0.46 Sandy clay loam 0.39 Clay loam 0.30

Mulching avocado trees

Mulching avocado trees

Overhead misters

Research needed Establish ‘Crop factors’ for different times in the annual growth cycle (thought to exceed 1.0 during flowering) More accurately determine what water tension levels result in ring neck, excess fruit shedding, death of seed coat, and other water stress effects Partial drying of the roots has been tried but results in crop loss Using water stress as a management tool only has negative effects

The water contained a red fluorescent dye – Water stress ‘memory’: induces avocado to block xylem with tyloses as a survival mechanism Un-watered Well-watered The water contained a red fluorescent dye – so red means water can flow through the vessels

Typical avocado irrigation system Tree spacing 9 x 4m Under-tree mini sprinklers (one to two per tree) Systems deliver 80 to 120 L/tree/hour About 60% of orchard floor is wetted Precipitation rates about 2mm/hour in wetted area Systems must be capable of meeting peak demand days Capacitance probes (e.g. Enviroscans), gypsum block and tensiometers are popular moisture monitoring tools. Capacitance probes typically measure hourly.

Acknowledgements