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Michael D. Meyer and Mary K. Hausbeck

The effects of bed height, mulches, composted poultry litter, and cultivars on Phytophthora crown rot, caused by Phytophthora capsici Leonian, of summer squash (Cucurbita pepo L.) were evaluated in the absence of fungicide applications. The experimental design was a split-split-split plot arrangement of a randomized complete block. Bed height (flat or raised) was the main plot treatment. Mulches (bare soil, wheat straw, or plastic) were subplot treatments. Composted poultry litter applications (0 or 4.5 t·ha−1) were sub-subplot treatments. Squash cultivars (Cougar or Payroll) were sub-sub-subplot treatments. Incidence of plant death (%) was assessed from 0 to 35 days post-inoculation (dpi) with P. capsici. Plant death 35 dpi and area under the disease progress curve (AUDPC)differed significantly (P < 0.0001) between the cultivars Cougar and Payroll. Mean plant death 35 dpi was 87% for ‘Payroll’ and 99% for ‘Cougar’. The bed height × cultivar interaction was also significant (P = 0.0018) in the analyses of variance for plant death and AUDPC. Plant death at 35 dpi and AUDPC for ‘Payroll’ were greater in flat beds than raised beds. Disease was unaffected by the main effects of bed height, mulch type, or application of poultry litter. Thirty-two summer squash cultivars and 10 germplasm accessions were also evaluated for resistance to Phytophthora crown rot in a separate greenhouse trial. Crown rot severity was rated on a 1 (no symptoms) to 5 (plant death) scale at 18 dpi. Crown rot severity differed significantly (P < 0.0001) among cultivars and germplasm accessions. Crown rot severity averaged 4.3 on commercial cultivars and 2.2 on germplasm accessions. Crown rot was least severe on the commercial cultivar Spineless Beauty (mean rating = 2.9). No disease developed on four accessions of Cucurbita moschata previously reported to be crown rot-resistant.

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Johanna Del Castillo Múnera and Mary K. Hausbeck

Root rot caused by Pythium spp. is a significant disease on greenhouse-grown crops and negatively affects the floriculture industry. To develop strategies that limit Pythium root rot on geranium and snapdragon, greenhouse trials were conducted to test plant protectants and cultivars. Seven fungicides and two biological control agents were evaluated on plants inoculated with P. aphanidermatum, P. irregulare, or P. ultimum. Disease severity was assessed using a scale of 1 (healthy, no disease symptoms) to 5 (plant death) from 5 to 45 day postinoculation (dpi). The area under disease progress curve (AUDPC) values differed significantly (P < 0.0001) among Pythium spp. and treatment. The interaction between plant protectants and Pythium spp. was also significant. For geranium, mefenoxam (trade name: Subdue Maxx) and Streptomyces lydicus WYEC108 (trade name: Actinovate) effectively controlled Pythium root rot. For snapdragon, treatment efficacy varied depending on the Pythium sp. When 11 geranium cultivars were inoculated with P. aphanidermatum, none were completely resistant to the pathogen. However, when P. irregulare was used to inoculate these geranium cultivars, Nano White Hybrid and Bulls Eye Cherry were similar to the uninoculated control. When 12 snapdragon cultivars were inoculated with P. aphanidermatum or P. irregulare, no cultivar was completely resistant to either pathogen, but Twinny White and Candy Showers Yellow were least susceptible to both. Integrating effective fungicides with less susceptible cultivars can limit disease caused by Pythium spp.

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Charles S. Krasnow and Mary K. Hausbeck

Phytophthora capsici annually threatens production of cucurbit and solanaceous crops. Long-lived oospores produced by the pathogen incite primary infection of susceptible plants when conditions are wet. Limiting the rot of winter squash and pumpkin (Cucurbita sp.) fruits is difficult due to the long maturation period when fruits are often in direct contact with infested soil. Genetic resistance to fruit rot is not widely available within Cucurbita sp.; however, age-related resistance (ARR) to P. capsici fruit rot develops in specific cultivars during maturation. The objective of this study was to evaluate the fruits of 12 cultivars of Cucurbita pepo, Cucurbita moschata, and Cucurbita maxima for ARR to P. capsici using a mycelial-plug inoculation method. All Cucurbita pepo and Cucurbita moschata cultivars displayed ARR; 7 days postpollination (dpp) fruits were susceptible, limited lesion development occurred on fruits 22 dpp, and lesions did not develop at 56 dpp. Disease developed on both Cucurbita maxima cultivars tested at 7, 14, 22, and 56 dpp. Firmness of fruit exocarps was measured with a manual penetrometer. Exocarp firmness of all cultivars increased during maturation; however, there was no correlation between firmness and disease incidence among cultivars at 22 dpp (R 2 = −0.01, P = 0.85). When fruits of cultivars expressing ARR at 22 dpp were wounded before inoculation, fruit rot developed.

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Charles S. Krasnow and Mary K. Hausbeck

Pythium root rot (Pythium sp.) is ubiquitous in Michigan greenhouses that produce herbaceous ornamentals, an industry worth $393 million in the state. Disease symptoms include stunting, flowering delay, root rot, and death. Fungicides that are highly effective against pythium root rot are limited, and pathogen resistance has been documented. The objectives of this study were to determine the sensitivity of Pythium irregulare, Pythium ultimum, and Pythium aphanidermatum isolates from symptomatic herbaceous greenhouse ornamentals to the fungicide etridiazole and to determine the influence of pH and etridiazole on Pythium mycelial growth and asexual reproduction. Isolates were tested in vitro for sensitivity to etridiazole by growing the pathogen on amended V8-agar plates sealed in plastic containers to minimize fungicide loss from the vapor phase. The majority of isolates of all three species were sensitive to the fungicide with EC50 (effective concentration resulting in 50% inhibition of linear growth) values ranging from 0.10 to 5.03 μg·mL−1. Two isolates of P. irregulare had an EC90 (effective concentration resulting in 90% inhibition of linear growth) value >80 μg·mL−1. The acidity of the medium influenced the ability of etridiazole to inhibit Pythium mycelial growth and asexual reproduction. Agar plates amended with 1 μg·mL−1 etridiazole and adjusted to pH 4.5 limited the mycelial growth of two P. aphanidermatum isolates and two P. irregulare isolates by 90% and 56%, respectively, compared with amended agar at pH 6.5. Sporangial formation by P. aphanidermatum was less frequent on mycelial disks incubated in etridiazole-amended sterile distilled water (SDW) at pH 4.5 than pH 6.5 (P < 0.05). P. aphanidermatum zoospore cyst germination was less sensitive to etridiazole than sporangia or mycelial growth; however, the influence of pH and fungicide on cyst germination was significant (P < 0.01). At 250 μg·mL−1 etridiazole and solution pH 4.5, zoospore cyst germination was inhibited 99.9% compared with 94.2% at pH 6.5. In a greenhouse experiment, disease symptoms were observed on ‘Pinto White’ geranium (Pelargonium ×hortorum) in a potting medium infested with P. aphanidermatum and adjusted to pH 4.5 or 6.5; however, plant health and fresh weight were greater in low pH potting medium. Etridiazole, applied as a drench at transplant, did not improve control of root rot for plants grown at low pH (P > 0.05). Fresh weight of plants grown in infested potting medium adjusted to pH 4.5 and amended with a single drench of etridiazole (100 μg·mL−1) was reduced 20%, statistically similar to the untreated control. Adjusting the acidity of irrigation water at the time of etridiazole application in ebb and flow and flood floor production systems could be beneficial in pythium root rot management of certain ornamental crops if plants have tolerance to low pH.

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Lina M. Rodríguez-Salamanca and Mary K. Hausbeck

Leaf and neck anthracnose is incited by Colletotrichum coccodes (Wallr.) Hughes, a new foliar disease of onion (Allium cepa L.) in Michigan that has been observed in the state since 2010. Symptoms include elliptical lesions on the leaves, necks, or both that appear bleached with a pale salmon to dark brown center. To develop an effective integrated disease management strategy, field studies were conducted in 2011 and 2012 to evaluate 16 commercial onion cultivars for their susceptibility to the pathogen. The incidence and severity of anthracnose were evaluated weekly following inoculation. Onion cultivars differed significantly in disease severity and incidence; differences between years were also observed. ‘Hendrix’ had the lowest disease severity, whereas ‘Highlander’ and ‘Candy’ exhibited severe onion leaf and neck anthracnose symptoms. Using less susceptible onion cultivars combined with effective fungicides against C. coccodes may limit crop losses for Michigan growers.

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Leah L. Granke, Layla E. Crawford and Mary K. Hausbeck

To determine the factors affecting airborne conidial concentrations of Podosphaera xanthii Braun and Shishkoff and powdery mildew severity in greenhouse-grown potted gerbera (Gerbera jamesonii H. Bolus), airborne concentrations of conidia were monitored in a glass and polyethylene greenhouse. Temperature, relative humidity, and leaf wetness were recorded onsite, and the percentage of foliage with visible disease was assessed weekly at the glasshouse and every 2 weeks at the polyethylene greenhouse (1 to 10 visual rating scale). Peak airborne conidial concentrations occurred at 0800/1600 and 0900/1400 hr at the glasshouse and polyethylene greenhouses, respectively. Few conidia were sampled between 2200 and 0500 hr at either greenhouse. Worker activity was associated with conidial release in the glasshouse, but not in the larger polyethylene greenhouse, and worker activity may have influenced the daily periodicity of conidial concentrations. Airborne conidial concentrations were not related to environmental conditions in the same hour as conidial detection. An increase in disease severity was positively related to relative humidity and negatively related to leaf wetness at both greenhouses; in addition, temperature was negatively related to an increase in disease severity in the glasshouse. In light of the results of this study, frequent scouting and fungicide applications for powdery mildew are advised. Wide plant spacing and adequate ventilation are also recommended to reduce relative humidity in the microclimate.

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Charles S. Krasnow, Rachel P. Naegele and Mary K. Hausbeck

Phytophthora blight is a destructive disease of cucurbits affecting the fruit, leaves, crown, and/or roots. Ten cucurbit PIs with known partial resistance to Phytophthora capsici root and crown rot were evaluated for resistance to Phytophthora fruit rot. Unwounded fruit from field-grown plants of Cucurbita moschata and C. pepo were inoculated in a controlled environment at 7 to 10 or 21 to 24 days post-pollination (dpp) with virulent P. capsici isolates to examine the effect of fruit age on disease development. Inoculated fruit were rated for lesion area and pathogen mycelial growth 7 days post-inoculation (dpi); fruit length, diameter, and pericarp thickness were also rated. Two C. pepo accessions (PI 169417 and PI 181761) had significant resistance to Phytophthora fruit rot at both 7 to 10 dpp and 21 to 24 dpp. All accessions evaluated displayed reduced disease susceptibility as the fruit aged.

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Mathieu Ngouajio*, Mary K. Hausbeck, Daniel M. Sullen, Mohan Selvaraj and Kevin Charles

Michigan is the national leader for pickling cucumber production. However, over the last few years growers have witnessed a considerable decline in marketable yield, mainly attributed to fruit rot caused by Phytophthora capsici. Phytophtora develops rapidly under high relative humidity, a situation commonly found with narrow rows. Growers are interested in using wider rows but would like to know if there are any associated yield reductions. This study was conducted in 2003 to measure the effects of cucumber plant populations on canopy dynamics and fruit yield. Cucumbers were grown with between-row spacing of 30.5, 45.7, 61.0, and 76.2 cm, and in-row spacing of 10.2, 12.7, and 15.2 cm. A split-plot design with four replications was used. Row spacing was the main plot factor, and in-row spacing the subplot factor. Soil covered by plant canopy was monitored throughout the growing season using digital image analysis techniques. At harvest, the number of fruits per plant and marketable yield for the different grades were measured. Cucumber canopy remained open during the major part of the growing season when wide rows (61.0 and 76.2 cm) were used. The number of fruits per plant increased from an average of 1.5 fruits at 30.5 cm to 2.0 fruits per plant at 61.0 cm. Further widening of row spacing to 76.2 cm slightly reduced the number of fruits per plant. Therefore, the optimum row spacing would be 61.0 cm if the number of fruits per plant was the only parameter being measured. Cucumber marketable yield was similar with 30.5, 45.7, and 61.0 cm spacing between the rows. With 76.2-cm rows, yield reduced slightly. These results suggest that cucumber plant density can be reduced substantial with limited yield penalty.

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Laura L. Arriola, Mary K. Hausbeck, John Rogers and Gene R. Safir

Commercially available biocontrol agents Trichoderma harzianum Rifai and the arbuscular mycorrhizal fungus Glomus intraradices Schenck and Smith were tested for their efficacy in controlling fusarium root rot in potted asparagus (Asparagus officinalis L.) seedlings. High and low concentrations of Fusarium oxysporum (Schlect.) emend. Snyd. & Hans. f. sp. asparagi Cohen & Heald (FOA) were combined with G. intraradices and/or T. harzianum treatments. In both experiments included in this study, T. harzianum and G. intraradices alone and in combination effectively reduced root rot caused by FOA when asparagus seedlings were grown in low levels of FOA-infested medium. When seedlings were grown in high levels of FOA-infested medium, the combination of T. harzianum + G. intraradices significantly increased dry shoot mass and limited root rot compared to the control.

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Charles S. Krasnow, Andrew A. Wyenandt, Wesley L. Kline, J. Boyd Carey and Mary K. Hausbeck

Phytophthora crown and root rot, incited by Phytophthora capsici, is an important and limiting disease in bell pepper (Capsicum annuum) production in many vegetable-producing areas of the United States. Soilborne oospores initiate disease when conditions are favorable, and polycyclic production of sporangia and zoospores occurs on infected plant tissue during the production season. Raised-bed plant culture, resistant cultivars, and oomycete-specific fungicides are commonly used to manage P. capsici. The objective of this study was to evaluate four bell pepper cultivars and four experimental breeding entries (collectively termed entries) for resistance to P. capsici in Michigan (MI) and New Jersey (NJ) and to determine the effect of a fungicide program on plant health and yield. The pepper cultivars included Camelot X3R (susceptible), Aristotle (intermediately resistant), and Paladin and Archimedes (resistant) for comparison. Disease symptoms included plant wilting and sunken necrotic stem lesions. In NJ, blighting of stems and foliage was also observed. In MI, >90% of the susceptible ‘Camelot X3R’ plants in the untreated plot wilted and died in both years of the study. All other entries had <10% plant wilting and death in 2014. In 2015, ‘Archimedes’ and ‘Paladin’ had <10% wilt and plant death; ‘Aristotle’, AP4835, 13SE12671, and AP4841 had 10% to 30% symptomatic plants. The fungicide program reduced disease to <10% for all entries except ‘Camelot X3R’ in 2014 and ‘Aristotle’ and ‘Camelot X3R’ in 2015. In NJ, ‘Paladin’, ‘Aristotle’, and ‘Camelot X3R’ (2014) and ‘Archimedes’, ‘Aristotle’, and ‘Camelot X3R’ (2015) had >30% plant wilting and death in the untreated plot. In the fungicide-treated plot, AP4841, AP4835, and AP4839 (2014), and AP4839 (2015) had <10% of plants with disease symptoms; ‘Camelot X3R’ and ‘Aristotle’ had >40% plant wilting and death in both years. In MI, marketable yield for ‘Paladin’ in fungicide-treated and untreated plots was significantly higher than the other entries in both years (P < 0.05). AP4839 was the highest yielding entry in NJ in the untreated plot, and AP4839 and ‘Archimedes’ were highest yielding in the fungicide-treated plot in 2014 and 2015, respectively. Fruit size for 13SE12671 was the largest among entries in both locations. There was no entry × fungicide program interaction in MI.