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Disease Risks

Pathogens are expected to react to climate change much more quickly than trees due to their mobility and much shorter life cycles. This will increase pathogen-induced forest disturbance.

Host – pathogen and host-vector-pathogen interactions may also be affected by climate induced changes to the synchronicity of codependent phenological and life-cycle events, potentially changing the severity and incidence of the disease.

Some of the likely specific impacts are:

  • Increased impacts, where host species are susceptible to pathogens when the trees are stressed (e.g., by water limitation), if further restrictions in water availability occur under climate change; but susceptibility may decrease if climate change improves water availability;
  • An increase in foliar disease impacts due to potentially increased humidity;
  • Shortening the reproduction period of the pathogen and thus increasing inoculums, resulting in increased disease prevalence. Conversely, in some circumstances, inoculums may be reduced and disease incidence could be reduced;
  • Changes to the rate of spread of a pathogen and its growth caused by changes in abiotic factors such as rainfall, temperature, humidity, wind, and radiation;
  • Changes in temperature and water availability affecting the ability of foliar fungi to penetrate the host and complete sporulation; 
  • A predisposition of trees to attack from pathogens under drought conditions;
  • Changes to pathogen reproductive capacity caused by changes in rainfall and relative humidity;
  • Surface wetness, temperature and high humidity influencing the process of infection and hence the success of spore infection;
  • Epidemics may become more frequent .

Climate change can have indirect effects on the epidemiology of both insects and pathogens through the effects of climate on other inter-related components of the ecosystems. This can increase the potential for disease syndromes that rely on insects as vectors.

A relevant example to New Zealand is that the distribution of Dutch Elm disease could change depending on the effects of climate on both the disease and also its beetle vector and pine canker distribution is dependent on its insect vectors.

​Diplodia

This fungus (Sphaeropsis sapinea syn. Diplodia pinea) is primarily a wound pathogen but can attack tissue under damp conditions. The impact on the current season’s growth is the dieback of terminal and lateral shoots. It can also infect stems leading to the death of part of the crown or even the whole tree. Uncommon direct infection of healthy stems has been recorded in Hawke’s Bay and Canterbury under conditions of severe drought. Climatic conditions of temperatures between 20°C and 25°C and >90% relative humidity favour the development of infection in green shoots. Hence,

With a warmer and drier climate the impact is expected to increase and under a warmer and wetter climate, the impact is not expected to change.

Armillaria

Armillaria species (Armillaria limonea and Armillaria novae-zelandiae) cause root disease resulting in wood decay, growth reduction and death, particularly in young radiata pine trees. Mortality in radiata pine occurs throughout the North Island and much of the South Island, being most prevalent on ex-native forest sites, but it also occurs at other locations. Armillaria occurs naturally in indigenous beech and podocarp & broadleaf forests. A. limonea is only known in the North Island and the northern South Island.

The incidence of this disease is expected to increase as temperature increases. With a warmer and drier climate the impact is expected to increase. Under a warmer and wetter climate, the impact is not expected to change.

Swiss needle-cast

Swiss needle-cast (Phaeocryptopus gaeumannii) disease affects Douglas fir  with symptoms include chlorosis, resulting in reduced retention of needles and growth reduction. It is distributed throughout New Zealand.

The climate variables that are positively correlated with infection level are a winter mean temperature above 5°C and infection is favoured if the spring/early summer is relatively wet.

Douglas fir is least affected on cooler, higher altitude, inland sites, especially in the South Island.

Under a warmer and drier climate, the impact is expected to not change. Under a warmer and wetter climate, the impact is expected to increase.

CLIMEX  has been used to predict the abundance of and severity of the disease, under climate change (A1B and A2 scenarios) by the 2080s, the suitable area in the North Island for Douglas Fir plantations would reduce from the current 100% to between 36% and 64%. This reduction was due to the increasing projected severity of Swiss needle-cast making many areas, at best, marginal for Douglas fir.

Pitch canker

Pitch canker (Fusarium circinatum) is not currently present in New Zealand. Under the current climate the northern and coastal areas of the North Island have a suitable climate for pitch canker. Under future climate scenarios, the majority of the North Island and northern and coastal areas of the South Island will potentially have suitable climate.

Dothistroma needle-blight

Dothistroma needle-blight is caused by Dothistroma septosporum and causes defoliation resulting in growth loss but rarely mortality. It is distributed throughout New Zealand, though with differing levels of severity.

Infection rates are dependent on diurnal temperatures falling between 5°C and 25°C. Summer rainfall is positively correlated with infection rates. Severe infection occurs with daily mean temperature of between 16°C and 20°C, a period of wetness of 10+ hours and where more than 2000 spores are present on a needle.

Under a warmer and drier climate the impact is expected to decrease or not change. Under a warmer and wetter climate the impact is expected to increase.

The results from the Climex model projections are that the potentially suitable areas will increase to the entire country under two of the GCM models; while the third model projects a small area in Southland remaining unsuitable for Dothistroma.

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