Keith S. Yoder

Glomerella leaf spot (GLS) and bitter rot caused by species in the Colletotrichum gloeosporioides species complex are the most economically devastating fungal diseases of apples in North Carolina. Crop losses have reached 100% in highly susceptible cultivars. In the southeastern United States, management of this disease has predominantly relied on broad-spectrum fungicide programs targeting several summer diseases of apples. To improve control of GLS and bitter rot, field experiments to evaluate the efficacy of multi- and single-site fungicides were conducted over multiple years in a ‘Gala’ research orchard in Mills River, NC, from 2017 to 2019. Fungicides representing different modes of action were applied in a nonrotational program from petal fall until the immediate preharvest period to assess their efficacy in reducing GLS incidence, shoot defoliation caused by GLS, and pre- and postharvest bitter rot. Of the fungicides evaluated, Captan 80WDG (active ingredient [a.i.] captan), Merivon (a.i. fluxapyroxad and pyraclostrobin), Cabrio (a.i. pyraclostrobin), and Aprovia (a.i. benzovindiflupyr) provided good to excellent control against disease caused by Colletotrichum chrysophilum. In these programs, the incidence of GLS, expressed as the relative area under the disease progress curve, ranged from 4.6 to 43.8 in the Captan 80WDG (2019) and the Aprovia (2017) programs, respectively. The incidence of bitter rot at harvest in these programs from 2017 to 2019 ranged from 0.0 to 45.3% in the 2018 and 2017 Captan 80WDG programs, respectively. Conversely, the highest incidences of GLS and preharvest bitter rot were consistently observed in the Inspire (a.i. difenoconazole), Sercadis (a.i. fluxapyroxad), and untreated programs. Shoot defoliation owing to GLS followed a similar trend across the evaluated fungicide programs. In addition to nonrotational fungicide programs, the effect of carrier water pH for applications of captan was evaluated for bitter rot control in Virginia and North Carolina. No differences in captan efficacy were observed at either location. The results of this study will inform the development of new management programs for GLS and bitter rot in the southeastern United States and provide a foundation for the development of new season-long fungicide programs for apple disease management.

Boxwood (Buxus L. spp. Buxaceae) are popular landscape plants in the United States, with traditionally low maintenance requirements, glossy evergreen foliage, and deer resistance. Buxus sempervirens ‘Suffruticosa' (English boxwood) and B. sempervirens (American boxwood) were the most popular Buxus taxa planted for hundreds of years. But in the 1970s and 1980s, ‘Suffruticosa' lost popularity due to boxwood decline, and many commercial nurseries began to search for new cultivars. However, many of the popular new varieties, such as ‘Justin Brouwers', ‘Green Beauty', ‘Green Mountain', and ‘Green Velvet' were very susceptible to boxwood leafminer (Monarthropalpus flavus Schrank). In 2011 boxwood blight, caused by the fungus Calonectria pseudonaviculata, was identified in the eastern United States and Oregon. Based on early reports that both leafminer and blight were variety-specific, Saunders Brothers Inc., a wholesale nursery in Virginia specializing in boxwood, began field trials to search for cultivars with natural resistance to boxwood leafminer and boxwood blight. Reported here are field evaluations of 146 cultivars in leafminer trials and 75 cultivars and selections in blight trials. In these trials, cultivars ‘Peergold', ‘Cole's Dwarf', ‘SB 108’, ‘SB 300’, and ‘Wee Willie' and selections SB17 and 9-00-174 had low susceptibility (resistance) to both blight and leafminer. Species used in this study: Buxus harlandii Hance; B. microphylla Seibold & Zucc.; B. microphylla var. japonica D. Anberg; B. sempervirens L.; B. sinica var. insularis (Nakai) M. Cheng; Calonectria pseudonaviculata (Crous, J.Z. Groenew. & C.F. Hill); L. Lombard, M.J. Wingf. & Crous, 2010; Monarthropalpus flavus (Schrank). Buxus nomenclature according to Batdorf, 2021.

Apple (Malus × domestica Borkh.) growers require management practices that will produce high-quality fruit while minimizing the number of chemicals used for adequate disease control and horticultural practices. Certain chemicals applied for bloom thinning also have fungicidal properties and could provide protection against early season diseases in addition to crop reduction. Over 5 years, treatments of lime sulfur (LS), Regalia (an organically approved biofungicide), and JMS Stylet-Oil (JSO) were evaluated for protection against apple scab (Venturia inaequalis [Cooke.] G. Wint.), powdery mildew (Podosphaera leucotricha [Ellis & Everh.] E. S. Salmon), cedar apple rust (Gymnosporangium juniperi-virginianae Schwein.), and quince rust (Gymnosporangium clavipes [Cooke & Peck] Cooke & Peck in Peck), as well as crop load reduction and fruit finish. Both LS and Regalia reduced apple scab and cedar apple rust in four out of five test years. Treatments of Regalia applied with JSO provided disease protection and crop load reduction similar to LS applied with JSO. We provide evidence that LS and Regalia, applied as bloom thinners, can reduce chemical applications used during bloom by combining two chemical spray functions: one for disease protection and one for crop load management.

In November 2014, leaves of ornamental pear (Pyrus calleryana, cultivar unknown) with bright orange to reddish spots typical of infection by a rust fungus were submitted to the Virginia Tech Plant Disease Clinic from a landscape in Frederick County, VA, for diagnosis. Brown aecia with bulbous bases resembling acorns, characteristic of European pear rust (pear trellis rust) caused by Gymnosporangium sabinae, were present on the lower leaf surface (Kern, 1973). Two ornamental pear trees that had been in the landscape for many years were heavily infected and scattered leaf lesions were observed on other ornamental pear trees in the neighborhood. Samples of infected leaves were sent to the USDA ARS Systematic Mycology & Microbiology Lab to confirm the presence of G. sabinae and for deposit in the U. S. National Fungus Collections (specimen number BPI 893287). Morphological characters were consistent with G. sabinae. Aecia were hypophyllous, in necrotic spots, roestelioid, 2–5 mm high, balanoid, splitting along sides with apex intact; peridial cells (n=20) sub-hyaline, verrucose to spinulose, elongated, 58–119 um × 10–22 µm; aeciospores (n=30) light brown to yellowish-brown, minutely verrucose, globose to ovoid, 22–29 um × 26–35 µm, with walls 3–5 µm thick. DNA was extracted from aecia on one representative leaf sample and the ITS2 and 5′ end of the 28S rDNA regions were amplified and sequenced using the primers Rust2inv and LR6 (Aime, 2006) (KU593568). Sequences were 99 – 100% identical to G. sabinae sequences in GenBank, including sequences from G. sabinae in New York (JN969962–JN96996). G. sabinae can infect leaves, fruit and young twigs and causes serious disease on ornamental pear and European pear (Pyrus communis). The alternate hosts of G. sabinae are in Juniperus section Sabina, but although a Juniperus species was present in the neighborhood, no signs of rust disease or cankers were found on any of the junipers. European pear rust has been reported elsewhere in the United States (CA, CT, MI, NY) and in Canada (BC); however, it has not previously been reported in Virginia. A report from Alabama could not be confirmed. Although European pear is not widely grown commercially in Virginia, it is grown as a backyard fruit tree, and ornamental pear is common in the landscape. G. sabinae can overwinter on both its pear and juniper hosts, increasing its potential for spread. Eastern red cedar (J. virginiana) is susceptible and commonly present, and invasive volunteer seeding of ornamental pears occurs within the region; therefore, we expect the rust fungus to spread to commercial and backyard ornamental and European pears. Because this rust can cause severe defoliation, reduction in yield and dieback of pear, as well as mummification of fruit (Prokopova, 2011), awareness of the presence of this disease is critical to its management on both ornamental and fruit-producing pears. This is the first report of European pear rust on ornamental pear in Virginia.

Apple scab is the most important disease of apples (Malus domestica) in the cool temperate regions where apples are grown. Data were collected over several years from plantings of new apple cultivars and selections established at multiple locations in 1995 (23 entries) and 1999 (24 entries) as part of regional project NE-183. Golden Delicious and McIntosh were the standards in both plantings. Data on incidence of scab on leaves and fruit were collected and analyzed to determine differences due to location and cultivar. None of the analyses indicated significant cultivar differences due to location, likely confirming the diversity of the apple scab fungus in the eastern United States. In general, for scab on leaves, the cultivars Ginger Gold, Orin, Cripps Pink, Silken, and Delblush were as susceptible as McIntosh, a common standard for scab susceptibility in the eastern United States. For scab on fruit, the cultivars Fuji, Cameo, Cripps Pink, Ambrosia, and Silken, and the selection NJ 90, were as susceptible as McIntosh. As expected, the cultivars and selections with genetic resistance to the scab pathogen exhibited the lowest incidences of apple scab on foliage and fruit. Gala Supreme, Sansa, and Honeycrisp, while susceptible to scab, exhibited relatively low levels of the disease. Accepted for publication 2 February 2010. Published 8 April 2010.

Effects of temperature and the combination of liquid lime sulfur (LLS) and fish oil (FO) applied during bloom on pollen germination and pollen tube growth in flowers and fruit set were examined in apples ( Malus × domestica Borkh.). Percent germination of pollen of ‘Manchurian’ crabapples and ‘Golden Delicious’ apple flowers on the stigmatic surface of ‘Golden Delicious’ pistils increased with increasing temperature from 13 to 29 °C in the first 24 and 48 h after pollination, respectively, but not thereafter. Pollen tube growth rate in the style increased quadratically with increasing temperature from 13 to 29 °C. ‘Manchurian’ was a more effective pollenizer of ‘Golden Delicious’ than was ‘Golden Delicious’ pollen. For example, at 24 or 29 °C, some ‘Manchurian’ pollen tubes grew to the base of ‘Golden Delicious’ styles by 24 h after pollination. On the other hand, no ‘Golden Delicious’ pollen tube grew to the base of a ‘Golden Delicious’ style regardless of temperature and time. Pollen tube growth rate in the style increased with increasing day/night temperature from 7/0 to 24/7 °C. The time required for pollen tubes to grow to the base of styles decreased with increasing day/night temperature from 13/2 to 24/7 °C. Only ≈36 h was required for pollen tubes to grow to the base of style at 24/7 °C, whereas pollen tubes grew very slowly and no pollen tubes grew to the base of style at 7/0 °C regardless of pollen source. LLS + FO, applied 4 or 24 h after pollination, inhibited pollen germination, pollen tube growth in the style, fertilization, and fruit set, but it had no effect when applied 48 h after pollination. These results suggest that LLS + FO applied at this bloom stage causes flower or fruit abscission most likely by inhibiting pollen germination, pollen tube growth in the style, and fertilization.