Mechanism of infection

The fruiting bodies (pseudothecia) cause a physical blockage (occlusion) of needle stomates, and this inhibits the uptake of carbon dioxide from the atmosphere for photosynthesis. When a threshold proportion of stomates are blocked (30-50%), the carbon cost of respiration exceeds the carbon gained through photosynthesis, and the needle becomes “carbon starved”. In these cases, it is beneficial for the tree to cast the needle. Researchers have also postulated that the reduction of carbon intake may also lead to the accumulation of damaging photo-oxidative chemicals within the needle (greater amounts of excess absorbed light). This helps to explain why more severe defoliation is observed in the upper crowns of diseased trees. Trees that have fewer than three years of needles are likely to experience moderate to significant growth loss compared to trees with longer needle retention.