Influence of calcium, potassium, and magnesium on Cornus florida L. density and resistance to dogwood anthracnose

Dogwood anthracnose, a major disease of Cornus florida L., has caused heavy mortality of C. florida in eastern United States forests. Disease severity and rate of infection have been shown to vary with several environmental factors, but the link between soil cation availability and anthracnose has not been examined. We hypothesized that soil cation availability, particularly calcium (Ca), potassium (K), and magnesium (Mg), would influence dogwood survival from anthracnose. In forested stands, positive correlations between soil Ca, K, and Mg saturation and C. florida stem density and basal area were found. The effect of these cations at four levels (0, 50, 100, and 200%) of a standard nursery fertilization rate on C. florida seedling survival and resistance to dogwood anthracnose was tested. Although most of the seedlings died after one season of exposure to dogwood anthracnose, seedlings that had lower inputs of Ca and K cations showed higher levels of disease severity sooner than seedlings in other treatments, suggesting that these nutrients play a role in C. florida survival from anthracnose. Magnesium treatment levels did not appear to have an effect on C. florida disease severity or mortality.     

The relationship between fire history and an exotic fungal disease in a deciduous forest

Exotic diseases have fundamentally altered the structure and function of forest ecosystems. Controlling exotic diseases across large expanses of forest has proven difficult, but fire may reduce the levels of diseases that are sensitive to environmental conditions. We examined Cornus florida populations in burned and unburned Quercus–Carya stands to determine if burning prior to anthracnose infection has reduced the impacts of an exotic fungal disease, dogwood anthracnose, caused by Discula destructiva. We hypothesized that fire has altered stand structure and created open conditions less conducive to dogwood anthracnose. We compared C. florida density, C. florida health, and species composition and density among four sampling categories: unburned stands, and stands that had burned once, twice, and 3 times over a 20-year period (late 1960s to late 1980s). Double burn stands contained the greatest density of C. florida stems (770 stems ha⁻¹) followed by triple burn stands (233 stems ha⁻¹), single burn stands (225 stems ha⁻¹) and unburned stands (70 stems ha⁻¹; P < 0.01). We observed less crown dieback in small C. florida trees (<5 cm diameter at breast height) in burned stands than in unburned stands (P < 0.05). Indicator species analysis showed that burning favored species historically associated with Quercus–Carya forests and excluded species associated with secondary succession following nearly a century of fire suppression. Our results suggest that fire may mitigate the decline of C. florida populations under attack by an exotic pathogen by altering forest structure and composition. Further, our results suggest that the burns we sampled have had an overall restorative effect on forest communities and were within the fire return interval of the historic fire regime. Consequently, prescribed fire may offer a management tool to reduce the impacts of fungal disease in forest ecosystems that developed under historic burning regimes.     

Fungal endophytes which invade insect galls: insect pathogens,benign saprophytes,or fungal inquilines?

Fungi are frequently found within insect galls. However, the origin of these fungi, whether they are acting as pathogens, saprophytes invading already dead galls, or fungal inquilines which invade the gall but kill the gall maker by indirect means, is rarely investigated. A pathogenic role for these fungi is usually inferred but never tested. I chose the following leaf-galling-insect/host-plant pairs (1) a cynipid which forms two-chambered galls on the veins of Oregon white oak, (2) a cynipid which forms single-chambered galls on California coast live oak, and (3) an aphid which forms galls on narrowleaf cottonwood leaves. All pairs were reported to have fungi associated with dead insects inside the gall. These fungi were cultured and identified. For the two cynipids, all fungi found inside the galls were also present in the leaves as fungal endophytes. The cottonwood leaves examined did not harbor fungal endophytes. For the cynipid on Oregon white oak, the fungal endophyte grows from the leaf into the gall and infects all gall tissue but does not directly kill the gall maker. The insect dies as a result of the gall tissue dying from fungal infection. Therefore, the fungus acts as an inquiline. Approximately 12.5% of these galls die as a result of invasion by the fungal endophyte.     

Sequence signatures from DNA amplification fingerprints reveal fine population structure of the dogwood pathogen Discula destructiva

Abstract Isolates of Discula destructiva Redlin and an undescribed species of Discula , the filamentous fungi that cause anthracnose of flowering ( Cornus florida L.) and Pacific ( Cornus nuttalli Aud.) dogwoods, were analyzed for genetic variation by nucleic acid scanning with arbitrary mini-hairpin oligonucleotide primers. While the fungal population was highly homogeneous throughout the disease range in eastern and western North America, the generation of arbitrary signatures from amplification profiles (ASAP) distinguished isolates from the northeast, middle and southern Appalachian mountain range, and western United States and Canada. ASAP involves a dual-step arbitrary primer-based amplification procedure that generates a large number of informative allelic characters by amplification of monomorphic DNA fingerprints. ASAP analyses showed a fine fungal population structure consistent with the recent and separate introduction of the pathogen on the west and east coasts of North America.