Drought, disease and red needle cast

The 2017/2018 summer has been a mixed bag of drought, tropical rain and high humidity. Drought conditions were recorded early in many normally well-watered parts of the country, sometimes just before flooding threatened to wash dry pastures away.

More short, seasonal droughts, and more extreme droughts in dry eastern areas, are expected with climate change.  

Droughts affect not only plants but also the pathogens that attack them. Predicting the possible effects of the double disaster of drought and pathogens on the country's agricultural, horticultural and forestry industries has been explored in a recent paper by scientists from Scion, Lincoln University, Landcare Research, the University of Auckland and the Auckland University of Technology.

The authors consider the potential impacts of drought on some diseases of plants important to New Zealand, including grapevine black foot (Ilyonectria/Dactylonectria spp.), kiwifruit sclerotinia rot (Sclerotinia sclerotiorum), and radiata pine red needle cast (Phytophthora pluvialis).

Increased drought is expected to increase disease expression for most pathogens. However, drought may reduce the severity of some diseases, such as sclerotinia rot of kiwifruit and red needle cast (RNC) of radiata pine.

Red needle cast, which causes needles to turn red, die and be cast, is very much on the mind of radiata pine growers. Levels of infection were particularly bad during the 2017 winter with wet conditions that favoured the spread of RNC. Douglas-fir is also affected.

Mireia Gomez-Gallego (PhD candidate in Forest Pathology) says spring was wet, and the start of summer very dry. The question of how the combination of drought and disease affect tree health and productivity is very timely.  

“We think trees’ susceptibility to RNC will be affected by speed of drought onset, intensity and duration.

“For example, when drought is gradual, trees respond by reducing transpiration, which reduces photosynthesis and hence carbon content in the leaves. Growth rate slows and trees also shed their needles. Low carbon reduces the ability of trees to fight infection, but on the other hand also makes for a poor food source for P. pluvialis, and fewer needles make it hard for the invading pathogen to take hold. We still need to test how drought affects the minimum carbon available for defense, and the minimum carbon in the needles to allow infection.

“In a typical scenario of a dry summer and seasonal onset of RNC in late autumn, outbreaks could be delayed until late winter due to the P. pluvialis population needing a longer period of wet conditions to build up organism numbers, spread and begin the infection cycle. But, when the outbreak takes place trees will be less prepared to fight it.”

The worst case scenario would be either drought or disease recurring in successive seasons. Growth will be slowed and productivity reduced.  

The changes in rainfall patterns likely due to climate change will affect the way trees and other crops important to New Zealand are affected by disease. As a country, we need to know more about how drought affects disease outcomes, including economic losses, to develop strategies for disease prevention and control.  

For the forestry industry, this is where research on controlling RNC through aerial spraying with copper, understanding the infection process and identifying and breeding resistant trees, becomes vital.

The news for many other diseases of importance to New Zealand’s primary sector is not as good. For most systems reviewed in the paper, drought is expected to lead to increased disease pressure. Coupled with moisture stress, this is likely to affect production in many areas. Unfortunately, we have no idea how serious the impact could be as there has been little research in the area. However, this will be important and the primary sectors need to be prepared for this.

Wakelin, S. A., Gomez-Gallego, M., Jones, E., Smaill, S., Lear, G., & Lambie, S. (2018). Climate change induced drought impacts on plant diseases in New Zealand. Australasian Plant Pathology, 1-14.

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