Common Garden Diseases in New Zealand

Many of us wish to have a lush and healthy garden full of trees, plants, vegetables and blooming flowers. But unfortunately, numerous plant diseases in New Zealand affect our environment. Fungi, bacteria or viruses cause them. 

Fungus disease is very diverse and can affect all plant parts, like the roots, stems, leaves, etc. Fungi, including black spots, rusts, moulds, and mildews, will attack various plants such as fruit trees, roses, vegetables and more. 

This article focuses on diseases that we most commonly see when completing tree work in Hamilton's residential areas.

Griselinia root rot caused by Phytophthora

Griselinia Root Rot Caused By Phytophthora

What is Phytophthora? 

Phytophthora is a type of root rot existing in the soil. If left untreated, it can cause the decay of roots in plants. In fact, moist soil is a suitable habitat for these fungi. So, it’s vital to have good drainage for the plant. When there is too much water near the roots, Phytophthora spores will travel and can reach the plant’s roots. 

It’s essential to add that phytophthora affects many other plants, not just Griselinia. 


The plant may become yellow or brown when infected with root rot. They can even fall off. These symptoms can worsen if left untreated. In addition, the plant could die because it won’t be drinking water properly. 


If you notice the symptoms listed above, we recommend testing the soil. If it comes back positive with the disease, you may want to consider treating the plant as soon as possible. 

  • Remove affected plants 

  • Replace the soil with fresh topsoil to prevent further spread 


We have seen more and more Griselinia diseases this year because it has been extremely dry. The plant roots get easily damaged because it doesn’t have enough energy to absorb from the dry soil. The Phytophthora will then attack the specie. 

Since this disease spreads fast, we recommend not planting this hedging shrub and looking for alternative species not susceptible to root rot, such as Buxus, Taxus, Ribes, etc.   

However, if you already have Griselinia hedges around your property, we recommend doing these steps to prevent the spread or formation of the root rot:

  • Increase drainage of the soil 

  • Improve the general soil’s health

  • Add mulch

Buxus Blight

Buxus Blight

What is Boxus Blight?

Buxus blight is a fungal disease caused by Cylindrocladium buxicola, affecting the Buxus species (Box). This disease doesn’t kill the roots, but it will weaken the plant. However, if you catch it early, recovery might be possible. 


This disease causes leaves to brown and fall off. In addition, it can infect young stems and cause black streaks. 


If the plant is infected:

  • Prune out the infected limbs

  • Sterilise your equipment between plants

  • Remove all leaves under the plants

  • Spray foliage with a fungicide called Buxus Blight Fighter


  • Check your plants for any signs of the fungal disease

  • Prune once a year as it will reduce the risk of spreading 

  • Ensure a good airflow around your plants

  • Water at the base of the plant 

  • Improve your soil’s health by adding mulch around the trees

Sooty Mould

Sooty Mould

What is sooty mould?

Sooty moulds are caused by Ascomycete fungi. They grow on the sap from plants and produce a sweet honeydew as a waste product discharged by insects like scale and aphids. The mould then expands on the honeydew. However, the mould doesn’t feed on the plant; it covers the leaf surface, blocking the light and reducing photosynthesis, vital for plant growth. 


The sooty mould forms a thick black coating on the tree's trunks, branches, and leaves. 


  • Spray with insecticide to control the sap-sucking insects

  • Spray with a fungicide to kill the mould 


  • Grow a variety of different plants to build an appropriate habitat for beneficial insects that predate on pests 

  • Create good airflow between plants 

These diseases aren’t always easy to find, so contact us for advice or information.


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