Tuesday, September 24, 2013

Skimpy Spruce – Defoliation from the Bottom Up

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. 

Figure 1 – Needles infected with Rhizosphaera turn purplish brown during summer.
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. 

Figure 2 – Needle drop and thinning of lower canopy are classic symptoms of Rhizosphaera needle cast in spruce.
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. 

Figure 3 – Fungal pycnidia are often visible without a hand lens.
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. 
Figure 4 – Fungal pycnidia protrude from stomata.