8/9/2023 0 Comments Needle blight in firThe key to preventing and managing this disease is keeping the needles as dry as possible. In areas where the disease is quite prevalent choose varieties of spruce that are more resistant, such as Norway or white spruce.For regions experiencing 2 infections per year the new growth will die and turn purplish brown and begins dropping from the tree by fall (University of Maryland Extension, (2021)). Under these moist conditions the ascospores penetrate needles and begin producing mycelium which will kill the needle by spring. This condition usually exists in regions that have rainy spring weather, but continuous overhead irrigation will also create the condition. In order for them to germinate the needles must remain constantly moist for 2-3 days. The ascospores can be splashed or blown onto the new growth or spread to new susceptible trees. In late spring, when the weather is warm and moist, the fruiting bodies of the fungus (pycnidia), begin erupting through the stomata and releasing ascospores. For areas experiencing only one infection period per year needles that were infected the previous season die by early spring and turn purplish brown. The fungus overwinters on needles that were infected the previous season, or dead needles, either as “(mycelium) inside needles and/or pycnidia on the outside of infected needles” (Thrush P., Taylor N. With some regions capable of experiencing 2 infections per year (University of Maryland Extension, (2021)). Rhizosphaera kalkhoffii has a one-year life cycle (Zeleznik J., Walla J., (Revised June 2019)). In severely infected trees, that have experienced repeated bouts of defoliation, only the new growth may remain on the tree. With each new infection inner needles continue to die off and drop from the tree making the tree appear thin, sparce and twiggy, especially towards the bottom and on the northern side. For regions that can experience 2 infections per year the new growth will turn purplish brown and begin dropping from the tree by fall (University of Maryland Extension, (2021)) otherwise they discolour by spring. On a healthy needle you will see fine white lines, which are the stomata (the breathing pours). They may or may not have a tiny white cap on their tops, which is the plug from the stomata pores. They appear as dark, smooth, tiny little balls that are arranged in fine rows up and down the needles. The mature pycnidia are very tiny, but they are readily visible with the aid of a hand lense. The fruiting bodies of the fungus (pycnidia) will begin emerging through stomatal pores on these discoloured needles and release spores that can be splashed or blown to new foliage. By spring these infected needles die and usually turn a purplish-brown, although they may also appear to be brown, reddish brown or tan in colour. To the untrained eye this initial infection is difficult to detect. The first symptom of infection is a faint yellow band or mottling on infected needles along with small immature fruiting bodies. To positively identify Rhizosphaera and its species samples will need to be sent to a laboratory for microscopic evaluation, as other needle cast diseases or conditions appear very similar. The species Rhizosphaera kalkhoffii is the one most encountered on spruce, especially Colorado Blue Spruce. Rhizosphaera is comprised of several species and are known to infect various species of a variety of conifers including Spruce ( Picea), True Fir ( Abies), Douglas Fir ( Pseudotsuga), Pine ( Pinus), Cedar ( Cedrus), Hemlock ( Tsuga) (Hudelson B., (Revised: )) and larch ( Larix) (Brazee N.J., (Updated: April 2022)).
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