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Husk Spot – A Review of What We Know

By Kevin Quinlan, Supply Chain Manager-NIS, MPC

With a new season about to commence, it is timely to review what we know about husk spot

Occurrence and infection

Sticktights – are the greatest source of inoculum in the orchard and infections on the husk can still produce viable spores for at least 2.5 years after they form;

Relative incidence of husk spot in trees with and without sticktights. From Drenth (2007)HAL Final Report -MC 03007 - Integrated Management of Husk Spot Disease (Pseudocercospora macadamiae) in Macadamias.

Relative incidence of husk spot in trees with and without sticktights. From Drenth (2007)HAL Final Report -MC 03007 – Integrated Management of Husk Spot Disease (Pseudocercospora macadamiae) in Macadamias.

Husk spot can only infect living (green) husk – but it can survive on dead husk and still produce spores. If the husk is composted, the husk spot fungus will die;

Stomata density –The only way husk spot fungus can enter the macadamia husk is through the stomata. Stomata are tiny “holes” that allow the plant to breathe or exchange carbon dioxide and oxygen, for photosynthesis and respiration. Stomata also allow transpiration, which supplies water and minerals to the entire plant system water. Stomatal density varies among cultivars. The higher the stomata density per unit area of husk, the greater the chance of infection;

Density of Stomata per unit area of husk of seven macadamia cultivars. From Drenth (2007)HAL Final Report -MC 03007 - Integrated Management of Husk Spot Disease (Pseudocercospora macadamiae) in Macadamias

Density of Stomata per unit area of husk of seven macadamia cultivars. From Drenth (2007)HAL Final Report -MC 03007 – Integrated Management of Husk Spot Disease (Pseudocercospora macadamiae) in Macadamias

Nut stage – nuts can be infected at anytime, but yield-limiting infection occurs from match-head to full nut size (roughly Late Nov/Early Dec);

Relative Humidity (RH) – The higher the Rh, the more favourable the conditions for husk spot infection to occur. The ideal condition for husk spot is free water on the husk surface from rainfall or dew at temperatures around 26°C;

A husk spot spore can adhere to the husk, germinate and grow through the open stomata within 20 hours.

Management to reduce yield loss

A history of husk spot in the orchard, sticktight nuts in tree canopy and varieties with high stomatal density are the three key factors that predispose an orchard to husk spot infection under favourable (warm and wet) conditions. These factors can help you to determine your risk of infection – and help with your preparedness;

Be prepared to apply your first fungicide treatment in spring/summer when nuts are match-head size and then again as prescribed on the fungicides label – especially if it’s warm and wet. If you use Cabrio® fungicide you can only spray two consecutive sprays with it and then you must use an alternative fungicide, such as Carbendizum. This is to minimise the risk of the husk spot fungus developing resistance to the product;

Good spray coverage is essential. Ensure you have well calibrated high volume spraying equipment and it achieves good nut coverage – as this treatment approach has been found to provide the best financial returns;

Monitoring the maturity of early drop nuts is a critical part of husk spot management. Many growers have found through maturity monitoring that product they considered premature and worthless at the beginning of the season has a value. Some have reduced their losses by as much as 40%. MPC offers free, rapid maturity monitoring for its suppliers. MPC accepts NIS that has as little as 18% usable kernel recovery (premium + commercial) and so with a highly husk spot susceptible variety like A16 with a total kernel recovery of around 38%, you would need to have very high level of reject before your product is unacceptable.

Overall it is the interaction of the predisposition factors, the stage of nut development and the weather that will dictate the level and risk of husk spot infection and the amount of nut drop at the beginning of harvest.

Husk Spot Decision Tree

The following diagrams are designed to assist you determine your risk level for husk spot infection. This will then allow you to determine what control strategies (if any) you need to apply.

There are 3 parts:

Part One is to determine the predisposition risk. That is, the level of risk your varieties, previous season, orchard layout etc pose to you having the disease;

Part Two is to determine the impacts of the seasonal conditions. Use the seasonal influence to determine if you are in a susceptible period of nut development and if the weather conditions are conducive to husk spot infection occurring.

Part 3 is used to combine the results from part one and two to give you an overall risk level for husk spot infection.

Part One

predisposition1a

Part Two

seasonal-factors1a
Once you have scored for predisposition and seasonal conditions influence,
use the table below to determine your likely outcome.

Part Three

final-score1a1