Rhizosphaera needle cast, the most common disease of spruce
in Kentucky, causes needle drop in lower branches, resulting in a distinct
thinned appearance. The fungal pathogen Rhizosphaera kalkhoffii primarily infects
spruce but can also affect some pine species.
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Figure 1 – Needles infected with Rhizosphaera turn purplish brown during summer.
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Symptoms are often noticed during summer when needles
on lower branches turn purplish or brown (Figure 1). Within a few weeks, needles fall and lower
limbs are left bare (Figure 2). Small,
dark fruiting bodies called pycnidia form in stomata (pores in needles) and can
be used to confirm diagnoses (Figures 3 & 4). Pycnidia can easily be recognized with a hand
lens or with the naked eye.
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Figure 2 – Needle drop and thinning of lower canopy are
classic symptoms of Rhizosphaera needle cast in spruce.
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The life cycle of the fungus extends over a 15-month
period. Infection takes place as spores
(conidia) within these pycnidia are rain splashed from needle debris onto
foliage. This infection process occurs
primarily during spring, but it can continue as long as conditions are rainy,
such as this past summer. During the
winter or following spring, pycnidia develop in infected needles, plugging stomata. Needle discoloration and needle drop occur during
summer of the second season, resulting in thinning of lower canopies. If defoliation occurs 3 to 4 consecutive
years, branch death is likely. Stressed trees
are more susceptible to infection by R.
kalkhoffii than healthy plants.
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Figure 3 – Fungal pycnidia are often visible without a hand
lens.
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Disease management should consist of good cultural
practices such as improved vigor and reduced plant stress, proper spacing to
improve air circulation, and most importantly, good sanitation habits. During rainy seasons or in plantings with a
history of disease, fungicides may be applied two consecutive years during
spring when fungi are most active. Fungicides that include chlorothalonil,
copper, or mancozeb are effective when applied during needle emergence
(mid-April) and again four weeks later. For
more information, see Homeowner’s
Guide to Fungicides.
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| Figure
4 – Fungal pycnidia protrude from stomata. |
