Jennifer Juzwik

Phytophthora spp. associated with forest soils in eastern and north-central U.S. oak ecosystems

A survey of soils associated with oak species was conducted in 2003 and 2004 in Indiana, Illinois, Maryland, Michigan, Minnesota, Pennsylvania, Ohio, West Virginia, and Wisconsin to investigate the occurrence of Phytophthora spp. Soils taken from around the base of healthy and declining oak trees were flooded with H2O and Quercus robur leaflets were used as bait for Phytophthora spp. From 829 soil samples collected near trees, 21% were positive for Phytophthora spp., with 55% of the 125 sites surveyed yielding a Phytophthora sp. Phytophthora cinnamomi was the most frequently isolated species, representing 69.4% of the Phytophthora-infested sites surveyed. Other species, in decreasing order of isolation frequency were Phytophthora sp. 2, P. citricola, P. europaea, P. cambivora, P. quercina-like isolates, and Phytophthora sp. 1. No significant association was found between the presence of Phytophthora organisms and site characteristics such as latitude, elevation, soil pH, or the crown condition of the trees. However, in P. cinnamomi-infested sites, a significant association was found with the deteriorating crown status of Q. alba and the presence of P. cinnamomi. The absence of P. cinnamomi above the 40°N latitude range also was noteworthy.

The Origin of Ceratocystis fagacearum, the Oak Wilt Fungus*

The oak wilt pathogen, Ceratocystis fagacearum, may be another example of a damaging, exotic species in forest ecosystems in the United States. Though C. fagacearum has received much research attention, the origin of the fungus is unknown. The pathogen may have been endemic at a low incidence until increased disturbances, changes in land use, and forest management created conditions favorable for disease epidemics. The host genus Quercus contains some relatively resistant species native to the United States, further supporting the hypothesis that the pathogen is native in origin. However, there are also many common, highly susceptible Quercus species--a characteristic typical of introduced pathogens. Most convincingly, studies have shown that the known populations of C. fagacearum have experienced a severe genetic bottleneck that can only be explained by a single introduction. The weight of evidence indicates that C. fagacearum is an introduced pathogen, with possible origins in Central or South America, or Mexico.

Challenges and Successes in Managing Oak Wilt in the United States

Oak wilt, caused by the fungus Ceratocystis fagacearum (Bretz) J. Hunt, is an important disease of oaks (Quercus spp.) in the eastern United States. It has been particularly destructive in the North Central states and Texas. Oak wilt is one of several significant oak diseases that threaten oak health worldwide. The significant gains made in our knowledge of the biology and epidemiology of this vascular wilt disease during the past six decades has led to development of various management strategies.

Fusarium species associated with rhizosphere soil and diseased roots of eastern white pine seedlings and associated nursery soil

Fusarium species isolated from necrotic roots of eastern white pine (Pinus strobus) seedlings in two nurseries included F. acuminatum, F. equiseti, F. oxysporum, F. oxysporum var. redolens, F. proliferatum, F. sambucinum, F. solani, and F. sporotrichioides. In addition, all but F. sambucinum were isolated from the rhizosphere; all, in addition to F. graminearum, were also isolated from nonrhizosphere soil. Fusarium oxysporum, F. oxysporum var. redolens, and F. proliferatum were the most prevalent taxa in roots and nonrhizosphere soil. These three taxa plus F. solani predominated in rhizosphere soil. Species prevalence differed by site and date of collection, e.g. F. proliferatum was present at only one site. At least seven species of Fusarium were associated with seedling root rot and their prevalence differed according to site and time of year.

Fusarium spp. and Pinus strobus seedlings: root disease pathogens and taxa associated with seed

Eastern white pine (Pinus strobus L.) seeds weresown in soil infested with Fusarium proliferatum, rootnecrosis developed on seedling roots, and F. proliferatumwas reisolated from symptomatic roots; thus, demonstrating thatF. proliferatum is pathogenic toeastern white pine seedlings. Soils infested with F.acuminatum or F. sporotrichioides resulted infew diseased seedlings. Seedlings with root rot generally showed reductions inseedling height. All Fusarium species tested wererecoveredfrom rhizosphere soil samples. Three seedlots of Pinusstrobus were examined for Fusarium infestation.Fusarium species were recovered from most seeds in twoseedlots. Fusarium proliferatum and F.sporotrichioides were the predominant species isolated. Additionalspecies not previously reported from P. strobus included:F. acuminatum, F. chlamydosporum,F. equiseti, F. graminearum,F. moniliforme, F. poae, F.polyphialidicum, F. heterosporum, F.sambucinum, and F. semitectum.

Multiple Ceratocystis smalleyi Infections Associated with Reduced Stem Water Transport in Bitternut Hickory

Hundreds of cankers caused by Ceratocystis smalleyi are associated with hickory bark beetle-attacked bitternut hickory exhibiting rapid crown decline in the north-central and northeastern United States. Discolored sapwood colonized by the fungus commonly underlies the cankers. Field studies were conducted to test the hypothesis that C. smalleyi infections cause vascular system dysfunction in infected trees. Fifty C. smalleyi inoculations made at 1.8 to 3.8 m in height on stems of healthy bitternut hickory trees (13 to 28 cm in diameter at 1.4 m in height) resulted in extensive canker formation and sapwood discoloration 12 to 14 months after treatment compared with water-inoculated and noninoculated controls. Sap flow velocity (midday) was significantly lower in the infected trees compared with that in the controls. Sap flow velocity also was inversely correlated with the proportion of bark area with cankered tissues and with tylose abundance in the youngest two growth rings. Tylose formation in current-year vessels associated with C. smalleyi infections is likely responsible for much of the water transport disruption. It is hypothesized that multiple stem infections of C. smalleyi and the resulting xylem dysfunction contribute to crown wilt development in bitternut hickory exhibiting rapid crown decline.

Three Colopterus Beetle Species Carry the Oak Wilt Fungus to Fresh Wounds on Red Oak in Missouri

Beetles in the family Nitidulidae can transmit the oak wilt fungus, Ceratocystis fagacearum, to fresh wounds on healthy oak trees, leading to infection and disease development. Historically, nitidulid beetles have not been considered important vectors of the pathogen in Missouri. Studies were conducted in the spring of 2005 and 2006 to determine frequencies of nitidulid beetle species contaminated with C. fagacearum visiting fresh wounds on red oak trees in central Missouri. Colopterus truncatus, C. niger, and C. semitectus were the most abundant species collected from fresh wounds and the only species found to be contaminated with Ceratocystis fagacearum. Of 230 beetles assayed for C. fagacearum, 23 yielded the fungus. Contamination frequencies were higher for beetles collected in April than May; no beetles collected in June were contaminated. We hypothesize that Colopterus truncatus, C. niger, and C. semitectus are principal nitidulid beetle vector species in Missouri during spring. The risk for pathogen transmission by these beetles appears to be greatest in April and least in June.

Seasonal Dispersal of the Oak Wilt Fungus by Colopterus truncatus and Carpophilus sayi in Minnesota

Sap beetles (Nitidulidae) are considered important overland vectors of the oak wilt pathogen, Ceratocystis fagacearum, in the north central United States. Colopterus truncatus and Carpophilus sayi are thought to be the principal sap beetle vectors in Minnesota. Field studies using windoriented funnel traps baited with aggregation pheromones of the insects were conducted during 2 years in east central Minnesota. The studies compared temporal flight dynamics of Colopterus truncatus and Carpophilus sayi from April through October, measured the proportion of dispersing adults of each species carrying viable pathogen propagules, and estimated the populations of contaminated dispersing beetles in oak (Quercus spp.) stands with and without oak wilt. Abundance of Colopterus truncatus peaked in either April or May, but the pathogen was most frequently isolated (20 to 45%) from beetles obtained from oak wilt sites from July through September. However, the highest contaminated insect population (CIP) generally occurred during April and May. Abundance of Carpophilus sayi peaked in October of both years, but the pathogen was most commonly isolated from beetles collected during May and June. The CIP was highest in June. The infective insect potential, as indexed by CIP, was greatest for both species in the spring and was greater for Colopterus truncatus than for Carpophilus sayi.

Acquisition of ophiostoma quercus and Ceratocystis fagacearum by nitidulids from O. quercus - Colonized oak wilt mats

Field experiments were conducted to determine whether the frequency of Ceratocystis fagacearum and Ophiostoma quercus propagule acquisition by nitidulids visiting oak wilt fungal mats is affected by the presence of O. quercus on the mats. Augmentation sprays with O. quercus were used to achieve different levels of mat colonization by that fungus. The extent of colonization by O. quercus 7 to 14 days after spraying was greatest for postmature mats with open cracks (>0.5 cm) observed on the spray date (P< 0.02). Specifically, all six subsamples assayed per mat for 16 of 18 open, O. quercus-sprayed, postmature mats yielded O. quercus. The frequency of nitidulids with O. quercus was also highest (83%) for those collected from O. quercus-treated, postmature mats that were open on the spray date. The frequency of nitidulids with viable C. fagacearum (88%), however, was also greatest for beetles from the postmature mats. Thus, O. quercus colonization did not affect vector acquisition of C. fagacearum from the mats. In addition, O. quercus vectored by nitidulids probably does not result in natural biological control of overland transmission of C. fagacearum at fresh wound surfaces on healthy oaks, as previously suggested.

Incorporation of tracers and dazomet by rotary tillers and a spading machine

Soil fumigant efficacy in forest-tree and ornamental nurseries depends on the tillage tool used for incorporation. Maximum depth and uniformity of incorporation of surface applied materials by three rotary tillers and a spading machine were compared in a loamy sand nursery using ceramic-sphere tracers (1-3 mm diameter) and dazomet (tetrahydro-3,5,dimethyl-2H-1,3,5-thiadiazone-2-thione) micro-granules. Depth of incorporation in the top 30cm of soil was evaluated by (1) recovery of incorporated spheres in 2cm increments, (2) biocidal activity in 6cm increments, and (3) cone resistance by 1.5cm increments to 45cm. Uniformity of incorporation was evaluated by sphere recovery and biocidal activity. Depths above which more than 95% of the spheres were recovered for the four implements were: 12.5cm, Kuhn and Fobro rotary tillers; 17cm, Northwest rotary tiller; 21cm, Gramegna spading machine. The spading machine produced a distribution of spheres through the soil profile closest to a uniform distribution compared with that produced by the three rotary tillers. Lettuce seed (Lactuca sativa L.) germination was inhibited in the upper 12cm in low and high dazomet rate treatments, indicating that all four implements effectively incorporated dazomet into that zone. Maximum depth (24cm) for total inhibition of germination was observed for the spading machine regardless of chemical rate. Cone index values showed the following maximum penetration: 14cm, Fobro rotary tiller; 22cm, Kuhn and Northwest rotary tillers; 27cm, spading machine. All three measures of depth show a distinct superiority of the spading machine when the chemical fumigant must reach depths greater than 18cm. Within transects across the width of the implement, variations of sphere counts among 5cm³ volumes were much larger for the rotary tillers than for the spade machine.