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;
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;
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.
Once you have scored for predisposition and seasonal conditions influence,
use the table below to determine your likely outcome.
Management of Trunk Canker
Management of Trunk Canker (Phytophthora cinnamomi) following major wet weather events
Article by Jim Patch – Agricultural Liaison Officer
Orchard soils throughout the harvest period of 2009 have been saturated for extended periods. This warm and wet soil environment has been ideal for the growth and development of phytophthora cinnamomi, a plant disease that is regularly seen in macadamia and avocado orchards in Australia ( Greek: Phyto – tophthora; meaning plant- destroyer).
The real cost to the Australian macadamia industry is not fully understood at present. This disease needs careful monitoring and management prior to the next warm dry spring period when excessive nut/leaf drop may be encountered from water/nutrient stressed trees.
What is Phytophthora cinnamomi?
P. cinnamomi is a water mould. It thrives in warm moist soil conditions. The mould growth consumes the host plant tissue (mainly plant roots) causing areas in the host plant that appear to be rotten e.g. root rot.
What does Phytophthera do to macadamia trees?
P. cinnamomi causes damage to the roots and collar area of macadamia trees. This damage weakens or kills the tree. Phytophthora produces many different types of spores throughout it’s lifecycle. Figure 1 shows the lifecycle of phytophthora. Wet, moist soils with low levels of oxygen favour the growth and reproduction of phytophthora and this is when the greatest level of infection can occur.
The symptoms of phytophthora are rarely seen in wet weather. This is because the damage to tree roots is not evident as there is plenty of soil moisture available and the tree is not suffering stress. Once the weather turns warm, the tree is actively trying to take up soil moisture but there is a small amount of a healthy root system left and so cannot take up enough water or nutrients for its needs. This causes the tree to stress and the symptoms become evident.
After a long period of wet weather and as we head into spring, the potential for phytophthora affected trees to be seen is very high.
Trees of all ages will have leaves that become yellow and chlorotic to brown in appearance (figure 1) and there may be excessive leaf shedding and bare canopy. Lesions may appear in the bark of the root and trunk area and may extend into the limbs. A common symptom is heavy suckering from the rootstock of a tree. Mature trees may become heavily stressed when soil moisture levels are reduced and excessive nut shedding may occur in warm/hot dry periods in spring and summer.
How does the mould spread throughout the orchard?
Phytophthora moves about by producing different types of spores. Depending on the spore type, spores have the ability to live for periods of between a few weeks to years. Spores may enter a tree through damage to the trunk, damage to exposed surface roots or through healthy roots in water logged soils. When P. cinnamomi has entered a host plant, it grows by producing microscopic filaments called mycelium. The mycelium may grow from one macadamia tree to the next through the intertwined roots of neighbouring trees. Movement may also come about by the movement of spores in runoff water. Spores may be transported on machinery or by other similar means throughout the orchard, or in surface water during rainfall events.
How can phytophthora be controlled in a macadamia orchard?
It is best to use multiple options for the control and protection of trees from phytophthora. A mix of the following will give you the best results.
Exclusion and sanitisation. Prevention is the best possible control strategy, although it can be difficult as very few macadamia orchard sites are now free from the disease. However grower due diligence may assist in protection of young macadamia trees, e.g. Only plant trees from a nursery that uses soilless potting mixes. Avoid potting mixes with a soil component which may harbour phytophthora. All potting mixes should be heat treated to kill pathogen spores. Divert surface water from neighbouring sites away from the orchard.
Orchard Design. Areas of the orchard site that may become water logged for extended periods should not be planted. Areas that may be marginal should be mounded to avoid the risk of tree roots being waterlogged. Ensure that water is not trapped within the planted tree rows by the mounds. Irrigation systems should be designed to avoid overwatering. This can be achieved by watering blocks together that have similar water requirements and water infiltration rates.
Organic Amendments and Mulching. The best method to protect established macadamia trees is to concentrate on creating a healthy soil environment in the orchard. A healthy orchard soil will have soil properties that favour good tree growth and do not favour the development of phytophthora. A healthy soil may protect the macadamia trees by suppressing P. cinnamomi through the activity of soil borne micro organisms such as fungi and bacteria.
P. cinnamomi appears to be suppressed in rainforests by high levels of organic material which can also be replicated in part in macadamia orchard soil. Organic amendments and mulching is recommended as mulching stimulates plant root development, increases nutrient uptake and decreases evaporation from the soil. Mulching also increases soil – water holding capacity, reduces water runoff, reduces rain drop splash, facilitates drainage, regulates soil temperature, and provides a high level of nutrients for soil microbes. The avocado industry struggles with the same pathogen. Their industry is advised to set up irrigation sprinklers so they do not spray directly onto the tree trunk and skirt the trees high enough to allow adequate air flow to create a dry under tree environment. As the pathogen requires moisture to replicate and waterlogged soils benefit its spread, orchards need to be well drained and not over irrigated. Phytophthora is inhibited by alfalfa meal, cotton waste, soybean meal, wheat straw, chicken manure and urea.
Apply organic amendments such as animal manures and compost to help improve soil structure and fertility. Nutrients in organic matter (especially nitrogen) break down slowly and are released over time in plant useable form. Ammonia and volatile organic acids released by decomposing organic matter kill Phytophthora and the residual organic matter stimulates competitive and antagonistic micro organisms in the soil. Avoid inputs of material which contain excessive levels of ammonia which can cause root damage. The addition of compost or mulches should not reduce the ability of moisture and air to enter into the soil and root system.
Organic matter may be increased by growing cover crops in the inter row. Cover crops provide nutrients, increase soil organic matter, improve aeration and provide food for soil organisms. Cover crops may reduce orchard temperatures by 1-2°C in summer and protect feeder roots near the surface from drying out or being effected from excessive heat. Sweet smother grass is presently being used very successfully in many macadamia orchards but there are many other grass species that macadamia growers may use to suit their growing region.
The use of chemical treatments to aid in the control of phytophthora has been found to be beneficial. These can be applied to give a fast response while other long term protection and prevention strategies such as improving soil health are carried out.
Research has shown that timing of any application of Phosphorous acid is critical. Tree roots do not receive nutrients whilst a tree is flushing as nutrients are moving up the tree towards leaves and twigs. When the flush has hardened, carbohydrates and other products start moving down the tree to the roots. This is the best time to apply your applications.
Always apply the chemical as directed on the label and follow all safety precautions outlined on the label.
Apply Phosphorous Acid (Phos.Acid) to the foliage when the leaf flush has just hardened in early spring and autumn.
Application rates are:-
400g/L Phos. Acid: Apply at a rate of 2.5mL to 3.0mL product per litre of water. Apply 7.5 to 10.0L of spray solution per tree.
600g/L Phos. Acid: Apply at a rate of 1.7mL to 2.0mL product per litre of water. Apply 7.5 to 10.0L of spray solution per tree.
620g/L Phos. Acid: Apply at a rate of 1.6mL to 1.9 mL product per litre of water. Apply 7.5 to 10.0L of spray solution per tree.
Do NOT apply to trees under severe stress or during hot weather.
Research work is presently being conducted on trunk application using Phosphorous (phosphonic) Acid(eg. Phospot 400® and Agri-Fos 600®). The results of this work to date have been encouraging.
The rate being used for bark painting in this experimental work is 20% acid, i.e. Agri-Fos 600® is 1L of product and 2L water. Phospot 400® is 1L of product and 1L water.
The penetrant Pulse® is also being added to this bark application to assist the translocation of the potassium phosphonate throughout the tree. The rate of Pulse is 20ml of Pulse in 1L of mix.
This mixture has been applied to the trunks by spraying from the ground to the first limbs of the trees. The use of Pulse for foliar applications is not recommended as it could cause leaf burn to trees.
Diversity and Management of Phytophthora in Southeast Asia. Drenth, A and Guest, D I. Australian Centre for International Agricultural Research. Canberra. 2004.
Soil Environmental Factors and Their Relationship to Avocado Root Rot. Menge, JA. and Marais, LJ. Department of plant Pathology, University of California, Riverside. CA92521.
Strategies to Control Phytophthora cinnamomi Root Rot of Avocado. Menge JA. Department of plant Pathology, University of California, Riverside.CA92521
Biological control of Phytophthora Root Rot of Avocado with Microorganisms Grown in Organic Mulches. Jefferson, L. da Costa, S, Menge, A Casale WL. Department of plant Pathology, University of California, Riverside, CA, USA. 2000
Phytophthora Root Disease. Griffith-Jones. 2001
What is phytophthera cinnamomi?
Tasmanian Department of Primary Industries, Parks, Water and Environment www.dpiw.tas.gov.au/inter/nsf/
Fruitgrowers’ Newsletter-p9, Spring 2007
Phytophthora in Macadamia, Akinsanmi, O and Drenth, A. Australian Macadamia Society Journal p49-50, July 2008.