Effect of Temperature, Leaf Wetness Period, and Cultivar Susceptibility on Boxwood Blight Disease Development and Sporulation.

Boxwood blight causes great losses to the boxwood nursery industry and landscapes in 30 states in the United States. Understanding the epidemiological factors governing disease development will be important for disease forecasting and design of best management practices. We evaluated the effect of leaf wetness period (lwp) and temperature on lesion development and sporulation on three boxwood cultivars under controlled conditions to develop predictive models for disease development. We conducted detached leaf assays at 18 to 27°C and various lwp with the cultivars Buxus sempervirens 'Suffruticosa' (highly susceptible), B. sempervirens × B. microphylla var. koreana 'Green Velvet' (moderately susceptible), and B. microphylla var. japonica 'Winter Gem' (less susceptible). Detached leaves were inoculated with 200 conidia in 50 µl of suspension and disease incidence was recorded at 3 to 13 days postinoculation (dpi). Cultivar, lwp, temperature, and most interactions significantly influenced disease development. A minimum of 5 h of leaf wetness was required for any disease. Lesion development increased most rapidly between 12 and 15 h and continued to increase to about 21 h of leaf wetness. Temperatures between 21 and 25°C were optimal for lesion development. There was about a 7-day lag between appearance of lesions and maximal incidence of sporulation. The two less-susceptible cultivars had fewer lesions than Suffruticosa under the same infection conditions; in addition, leaf lesions of Winter Gem exhibited delayed sporulation and sporulation from a smaller proportion of symptomatic leaves. Response surfaces were developed for each cultivar to predict the disease incidence using the lwp and dpi. Our findings will help refine disease forecast models to improve management of boxwood blight.

The Identification of a New Species, Diaporthe humulicola, a Pathogen Causing Diaporthe Leaf Spot on Common Hop.

Common hop, Humulus lupulus, is a commercially important crop in the United States, with an increasing number of hop yards being established in the Northeast. In 2018, a new fungal disease was observed at two research hop yards in Connecticut. This new pathogen affected all hop cultivars being grown and caused leaf spots and browning of cones. The causal organism was isolated and Koch's postulates were performed to confirm pathogenicity. The disease symptoms were similar to the previously described Phoma wilt; however, morphological and phylogenetic analyses placed the causal organism as a new species of Diaporthe. We propose the name Diaporthe humulicola. The disease increased under hot, humid conditions (around 24°C and 90% relative humidity), which prevail during the summer in the northeastern United States as well as other parts of the country. An in vitro preliminary assessment of fungicide sensitivity revealed that pyraclostrobin and boscalid inhibited D. humulicola growth in culture and should be further assessed for field efficacy against this new disease of hop. The proper identification and monitoring of this pathogen will be important to inform hop growers of this new threat.

One Clonal Lineage of Calonectria pseudonaviculata Is Primarily Responsible for the Boxwood Blight Epidemic in the United States.

Boxwood blight caused by Calonectria pseudonaviculata and C. henricotiae is destroying cultivated and native boxwood worldwide, with profound negative economic impacts on the horticulture industry. First documented in the United States in 2011, the disease has now occurred in 30 states. Previous research showed that global C. pseudonaviculata populations prior to 2014 had a clonal structure, and only the MAT1-2 idiomorph was observed. In this study, we examined C. pseudonaviculata genetic diversity and population structure in the United States after 2014, following the expansion of the disease across the country over the past 5 years. Two hundred eighteen isolates from 21 states were genotyped by sequencing 11 simple sequence repeat (SSR) loci and by MAT1 idiomorph typing. All isolates presented C. pseudonaviculata-specific alleles, indicating that C. henricotiae is still absent in the U.S. states sampled. The presence of only the MAT1-2 idiomorph and gametic linkage disequilibrium suggests the prevalence of asexual reproduction. The contemporary C. pseudonaviculata population is characterized by a clonal structure and composed of 13 multilocus genotypes (SSR-MLGs) unevenly distributed across the United States. These SSR-MLGs grouped into two clonal lineages (CLs). The predominant lineage CL2 (93% of isolates) is the primary contributor to U.S. disease expansion. The contemporary U.S. C. pseudonaviculata population is not geographically subdivided and not genetically differentiated from the U.S. population prior to 2014, but is significantly differentiated from the main European population, which is largely composed of CL1. Our findings provide insights into the boxwood blight epidemic that are critical for disease management and breeding of resistant boxwood cultivars.

Phytophthora abietivora, A New Species Isolated from Diseased Christmas Trees in Connecticut, U.S.A.

A number of fir species (Abies) are produced as Christmas trees around the world. In particular, Fraser fir (Abies fraseri (Pursh) Poir.) is popular as it yields high-quality Christmas trees in temperate North America and Europe. A Phytophthora sp. causing root rot on Fraser fir was isolated from a Christmas tree farm in Connecticut, U.S.A., and found to be new to science according to morphological and molecular phylogenetic analysis using multilocus DNA sequences from ITS, Cox1, ß-Tub, Nadh1, and Hsp90 loci. Thus, it was described and illustrated as Phytophthora abietivora. An informative Koch's postulates test revealed that P. abietivora was the pathogen causing root rot of Fraser fir.

Managing nematodes without methyl bromide.

Methyl bromide is an effective pre-plant soil fumigant used to control nematodes in many high-input, high-value crops in the United States, including vegetables, nursery plants, ornamentals, tree fruits, strawberries, and grapes. Because methyl bromide has provided a reliable return on investment for nematode control, many of these commodities have standardized their production practices based on the use of this chemical and will be negatively impacted if effective and economical alternatives are not identified. Alternative control measures based on other chemicals, genetic resistance, and cultural practices require a greater knowledge of nematode biology to achieve satisfactory results. Here, we provide an overview of nematode management practices that we believe will be relied upon heavily in U.S. high-value crop production systems in a world without methyl bromide. Included are case studies of U.S. high-value crop production systems to demonstrate how nematode management practices other than methyl bromide may be incorporated.

Influence of Ammonium Sulfate and Rotation Crops on Strawberry Black Root Rot.

The combined effects of rotation crops and nitrogen fertilizers were examined on the strawberry black root rot disease complex. In July 1995, microplots were filled with soil that had a history of strawberry black root rot and seeded with two types of oats (Avena strigosa 'Saia oats' or A. sativa 'Garry oats') or with sorgho-sudangrass (Sorghum bicolor × S. sudanense 'Triple S'). Microplots planted with 1-year-old 'Honeoye' strawberry crowns served as the controls. In May 1996, the crops were chopped and incorporated into the soil. The soil was re-planted with 1-year-old strawberry 'Honeoye' crowns and then fertilized with (NH4)2SO4 or Ca(NO3)2 at equivalent rates of N. Two months later, (NH4)2SO4-treated plants had 36% more leaf area and 41% more runners than strawberries treated with Ca(NO3)2 Strawberries that had been pre- cropped with 'Saia' oats had 135% more runners and 38% more early fruit yield than strawberries grown in control microplots. Total fruit yield was not affected by the treatments. Compared to Ca(NO3)2, the (NH4)2SO4 treatment reduced the percentage of blackened roots. The influence of the cover crops on growth and disease was stronger with (NH4)2SO4 fertilization than with Ca(NO3)2 fertilization. Combining 'Saia' oats or sorgho-sudangrass rotation with (NH4)2SO4 fertilization reduced lesion nematode (Pratylenchus penetrans) numbers in subsequent strawberry roots when compared to controls. Also, the combination of 'Saia' or 'Garry' oats as a pre-crop with applications of (NH4)2SO4 reduced black root rot severity when compared to controls. Only the 'Garry' oat rotation reduced strawberry root colonization by Rhizoctonia fragariae when compared to controls. Other effects were associated with using (NH4)2SO4. The (NH4)2SO4 treatment lowered the rhizosphere soil pH by 0.2 units, reduced the numbers of fluorescent pseudomonads in the rhizosphere by 10- to 15-fold, and produced leaves that had more N, K, S, Mn, and Zn content than plants treated with Ca(NO3)2. Rotation with 'Saia' oats combined with (NH4)2SO4 fertilization may suppress strawberry black root rot and increase yields through multiple effects on the host, pathogens, and associated microflora.