Tip Dieback Research Articles

Determining Available Soil Phosphorus in Nova Scotia Blueberry Soils

A project was initiated to determine the best predictor of plant-available phosphorus (P) for the Nova Scotia lowbush blueberry (Vaccinium angustifolium Ait.) from the following extractants: Mehlich 1, Mehlich 3, Bray 1, modified Morgan, and the anion exchange membrane. The study was initiated in 1999 with three newly established sites; two loamy soils located in South Branch (South Branch and White Fields) and one sandy loam soil in Debert. An established long-term experiment located at the Nova Scotia Wild Blueberry Institute on a sandy loam soil was also used. Soil samples were taken in the spring and at tip dieback (July) or harvest (August) from all sites during the 1999 and 2000 growing seasons. Tissue samples were obtained, with the July and August soil samples, to enable correlation of extracted soil P with the P present in the tissue. Results indicate that at all sites, except the White Field site, correlations between the five extractants and plant tissue were not significant in either year. However, correlation between the individual extraction methods at each site indicates that, with the exception of the South Branch site, all correlation coefficients were significant in both years. These conflicting results show that it would be misleading if a soil P testing program for blueberries was based solely on comparison with a previous soil P extraction method. This would result in a program that was incorrectly predicting the P requirements of the blueberry. Further work on the uptake of P by the blueberry plant under conditions of low and high levels of extractable P is required.

First Report of Blueberry scorch virus on Highbush Blueberry in Connecticut and Massachusetts.

During June and July 2001, the Marucci Center received 33 foliage samples from healthy- and unhealthy-looking highbush blueberry (Vaccinium corymbosum L.) bushes from growers in Connecticut and Massachusetts, via local extension agents. Unhealthy bushes were reported to exhibit symptoms including leaf chlorosis and necrosis, blossom blight, tip dieback, or a general decline in vigor. Marginal leaf chlorosis, reddening, or necrosis characterized foliage samples from these bushes. Five-leaf samples from each bush were tested for Blueberry scorch virus (BlScV) (3) with Agri-Check detection kits (Hydros, Inc., Falmouth, MA). These kits use an indirect enzyme-linked immunosorbent assay (ELISA) protocol and antibodies developed at Rutgers University that are specific to the two eastern strains of BlScV (NJ1 and NJ2) (1). The ELISA extraction buffer was based on that used by Martin and Bristow (3) with 1% (wt/vol) nonfat dry milk powder added as a blocking agent. Fourteen samples from cvs. Blueray and Berkeley in both states and cvs. Elliott, Bluecrop, and Coville in Massachusetts tested positive for BlScV. These results were confirmed by a second test. Six of seven samples from symptomatic bushes and 8 of 26 samples from asymptomatic bushes harbored BlScV. Virus preparations extracted from five infected plants (two from Connecticut and three from Massachusetts) were examined using reverse transcription-polymerase chain reaction (RT-PCR) with oligonucleotide primers (5'-TGTGTCAAACAATATGGC-3' and 5'-GCATTTCGATGA-TTGCGG-3') designed to amplify a portion of the coat protein gene from any of the three known virus strains (1,2). Sequence analysis of fragments amplified from their coat protein genes revealed that two of the isolates from Massachusetts (GenBank Accession Nos. AY530957 and AY530958) and the two isolates from Connecticut (GenBank Accession Nos. AY530955 and AY530956) were similar but not identical to one another, and these four were most similar to strain NJ2. One isolate from Massachusetts (GenBank Accession No. AY530958) was most similar to strain NJ1. To our knowledge, this is the first report of BlScV on the east coast outside of New Jersey, where it was first reported in 1983 (4). These results indicate that the disease is now present in other blueberry-growing areas in the northeast and is likely to be spreading locally within those areas. Because blueberry scorch is symptomless in propagation material and may take several years to express symptoms in the field, the initial spread of the disease was probably due to the shipping of latently infected plants to BlScV-free areas before reliable testing was available.

Determining Available Soil Phosphorus in Nova Scotia Blueberry Soils

Abstract A project was initiated to determine the best predictor of plant-available phosphorus (P) for the Nova Scotia lowbush blueberry (Vaccinium angustifolium Ait.) from the following extractants: Mehlich 1, Mehlich 3, Bray 1, modified Morgan, and the anion exchange membrane. The study was initiated in 1999 with three newly established sites; two loamy soils located in South Branch (South Branch and White Fields) and one sandy loam soil in Debert. An established long-term experiment located at the Nova Scotia Wild Blueberry Institute on a sandy loam soil was also used. Soil samples were taken in the spring and at tip dieback (July) or harvest (August) from all sites during the 1999 and 2000 growing seasons. Tissue samples were obtained, with the July and August soil samples, to enable correlation of extracted soil P with the P present in the tissue. Results indicate that at all sites, except the White Field site, correlations between the five extractants and plant tissue were not significant in either year. However, correlation between the individual extraction methods at each site indicates that, with the exception of the South Branch site, all correlation coefficients were significant in both years. These conflicting results show that it would be misleading if a soil P testing program for blueberries was based solely on comparison with a previous soil P extraction method. This would result in a program that was incorrectly predicting the P requirements of the blueberry. Further work on the uptake of P by the blueberry plant under conditions of low and high levels of extractable P is required.

First Report of Anthracnose Caused by Colletotrichum gloeosporioides on Taxus mairei in Taiwan.

Taxus mairei (Lemee & Levl.) S.Y. Hu ex Liu is a giant evergreen tree native to Taiwan. T. mairei and the Pacific yew, T. brevifolia, produce taxol, a highly effective antitumor drug. Anthracnose was observed on cuttings and seedlings of T. mairei in nurseries and on larger plants grown in plantations in Taiwan. Circular or irregular, brown leaf spots were associated with defoliation. Stems lesions and tip dieback were also observed. Colletotrichum gloeosporioides (Penz.) Sacc. was isolated from diseased tissues, and this fungus grew well on potato dextrose agar and malt extract agar (MEA), with a growth rate of 6.3 mm per day on MEA at 32°C in the dark. Colonies were white to grayish white and became dark gray with age. Acervuli produced on leaves were 115 to 155 μm in diameter, with one or several brown-to-black setae and pale salmon conidial masses. Conidia were guttulate, straight, cylindrical, obtuse at the apex and truncate at the base, and approximately 12 to 17 × 3.5 to 6.0 μm. The disease was reproduced by spraying T. mairei seedlings with a suspension of 104 to 105 conidia per ml, and the control plants were inoculated with distilled water. Inoculated plants were kept in a transparent moist chamber with a constant humidity near 100% (1). Symptoms appeared within 7 days when the temperature was over 32°C. When the temperature was below 24°C, symptoms were delayed. The fungus was reisolated from the inoculated plants, fulfilling Koch's postulates. To our knowledge, this is the first record of anthracnose on Taxus mairei (2), and anthracnose appears to be the most important disease on this plant in Taiwan. Voucher specimens have been deposited at the Division of Forest Protection, Taiwan Forest Research Institute as TFRIFCH Herbarium specimen 137.

First Report of a Leaf Blight of Onion Caused by Xanthomonas spp. in Georgia.

In October of 2001 and 2002, a leaf blight was reported affecting Vidalia onion (Allium cepa) cvs. Pegasus and Sweet Vidalia, respectively, in one field each. Lesions on onion seedlings began as a water-soaked, tip dieback that gradually blighted the entire leaf. Symptoms on onion transplants appeared as elongated, water-soaked lesions that typically collapsed at the point of initial infection. In both cases, disease was very severe on seedlings, and disease incidence was 50% or more in both fields. Warm temperatures combined with overhead irrigation and above average rainfall likely enhanced the severity and spread of disease. Disease was not detected on more mature onions once cool, dry conditions occurred later in the season, and no significant economic loss occurred. Seed was tested from seed lots of the aforementioned cultivars and Xanthomonas spp. were not found. Diseased tissue was macerated in sterile, phosphate-buffered saline, and 10 μl of the resulting suspension was streaked on nutrient agar plates. Yellow-pigmented, gram-negative, rod-shaped bacteria were isolated routinely from diseased tissue. Bacteria were catalase-positive, cellulolytic, oxidase-negative, amylolytic, proteolytic, and utilized glucose in an oxidative manner. Analysis of whole cell, fatty acid methyl esters (FAME) using the Microbial Identification System (MIS, Sherlock version 3.1; MIDI, Inc., Newark, DE) identified four representative strains of the bacterium as a pathovar of Xanthomonas axonopodis (similarity indices 0.75 to 0.83). Known Xanthomonas spp. from onion from Colorado and Texas (1,2) had similar FAME profiles when analyzed by the MIDI system. Onion plants were grown under greenhouse conditions for 2 months and inoculated by injecting the base of a quill with 1.0 ml of bacterial suspensions (1 × 107 CFU ml-1) of the Xanthomonas sp. isolated from Georgia, and negative controls were inoculated with 1 ml of sterile water. Disease symptoms developed on plants inoculated with bacterial suspensions in 4 to 7 days and Xanthomonas sp. was isolated from the lesions produced. Disease symptoms occurred when the same suspension was sprayed on onion foliage. No symptoms occurred on plants inoculated with 1 ml of sterile water. To our knowledge, this is the first report of Xanthomonas spp. affecting Vidalia onions.

Early Growth Characteristics of Seven Grafted Varieties and Non-Grafted Seedling Pawpaw

Following spring planting in 1998, and for four seasons (1998-2001), early growth and development were characterized for seven grafted [PawPaw Foundation (PPF), Frankfort, Ky.) pawpaw Regional variety trial (PRVT) rootstock] pawpaw (Asimina triloba) varieties, `Davis', `Taylor', `Rebecca's Gold', `Sunfl ower', `Overleese', `Prolific', `Wells', and one nongrafted seedling strain. Survival rate after four seasons was 96.5% for grafted trees and 100% for seedling trees. Entries exhibited significant differences in total height, trunk cross-sectional area (TCSA), average shoot length, first winter cold injury and percent bloom. By the end of 2000, seedling tree height and TCSA were greater as compared to the named varieties. Seedling height averaged 134 cm (52.8 inches), with a TCSA of 4.54 cm2 (0.704 inch2), while the least vigorous variety (Davis) had an average height of 77 cm (30.3 inches) and a TCSA of 1.54 cm2 (0.239 inch2). Average shoot length was significant only in the year of planting, ranging from 39.3 cm (15.47 inches) for `Davis', to 61.3 cm (24.13 inches) for `Sunfl ower'. Tip dieback occurred in the winter following planting on all varieties; this damage ranged from 23.3% (`Taylor') to 73.4% (`Overleese'). In 1999, lower axillary shoots (LSL) breaking below the damaged tips grew longer [31.1 to 50.6 cm (12.24 to 19.92 inches)] than upper distal axillary shoots (DASL) [15.8 to 44.6 cm (6.22 to 17.56 inches)], indicating a lack of apical dominance and vigor of emerging top buds. All named varieties developed fl ower buds by the spring of the fourth season, with budding ranging from an average of 0.5 buds/tree (`Rebeccas Gold') to 6.0 buds/tree (`Prolific'). Viability of these buds as determined by successful anthesis ranged from 0.4% (`Davis') to 63.8% (`Wells'). No fruit were set from these flowers.

First Report of Phytophthora ramorum on Douglas-Fir in California.

Phytophthora ramorum S. Werres & A.W.A.M. de Cock was isolated from three Douglas-fir (Pseudotsuga menziesii) saplings in a mixedevergreen forest in Sonoma County, California. Symptoms on these saplings included cankers on small branches (0.5 to 1 cm in diameter) resulting in wilting of new shoots, dieback of branches, and loss of leaves as much as 15 cm from the twig tip. Symptoms were observed on most saplings growing in the same area. On several smaller saplings (<1 m tall), P. ramorum infection resulted in the death of the leader and the top several whorls of branches. Isolates were identified as P. ramorum by their abundant chlamydospores and caducous, semi-papillate sporangia (2) and internal transcribed spacer rDNA sequences identical to those of isolates of P. ramorum from Quercus spp., Lithocarpus densiflorus, and Rhododendron (1,2). To test for pathogenicity, foliage inoculations were conducted on seedlings in two trials by misting 30 leaves per trial (five leaves per seedling plus controls) with sterile distilled water and pinning inoculum plugs, taken from the margin of P. ramorum cultures, to the upper surface of leaves. Inoculation resulted in lesions ranging between 1 and 12 mm long, and P. ramorum was recovered from 47% of inoculated leaves. Symptoms were not restricted to inoculated leaves, and in 26 single-leaf inoculations, lesions 17 to 85 mm long developed on branches (five mm in diameter) adjacent to the inoculated leaf. Isolation success from branch lesions was 50%, despite the fact that such lesions were apparently disjunct from the small 1-mm lesions developing on inoculated leaves. Stems of Douglas-fir seedlings (approximately 1 cm in diameter) were wound inoculated (1) in two trials consisting of 10 inoculated seedlings per trial plus 10 controls. After 6 weeks, lesion lengths in the cambium averaged 38 mm (range 12 to 62 mm), and three seedlings were completely girdled. P. ramorum was recovered from 75% of inoculated stems. Mean lesion lengths on seedlings inoculated with P. ramorum were significantly greater (P < 0.05) in both trials than those of control inoculations (mean 9 mm) based on analysis of variance. We have not observed unusual mortality or disease symptoms on overstory Douglas-fir trees in natural forests. The importance of P. ramorum branch tip dieback for growth and reproduction of Douglas-fir is unknown. Douglas-fir is present in many forests in California and Oregon already infested by P. ramorum, yet we have found infection of plants at only one location. At this site, symptomatic Douglas-fir saplings were surrounded by bay laurel (Umbellularia californica) trees with extremely high levels of P. ramorum infection. P. ramorum is known to sporulate prolifically on bay laurel leaves. More studies are necessary to determine if the incidence of P. ramorum in Douglas-fir extends to other locations or if it is limited to this one locale.

Bacterial canker, caused by Clavibacter michiganensis ssp. michiganensis, on tomatoes in eastern Anatolia region of Turkey

In 2001, a disease occurred on tomato (Lycopersicon esculentum) cv. Target growing in six different commercial fields in the production areas of Oltu, Ispir and Yusufeli in the eastern Anatolia region of Turkey. Disease incidence was almost 100%, resulting in heavy losses. Initial symptoms were small (1–3 mm in diameter), water-soaked, dark brown to black lesions on the leaf margins and stems, and white, raised ‘bird’s eye’ spots on fruits. Defoliation, one-sided wilting, tip die-back, open stem cankers and vascular discoloration were observed on plants in advanced stages of the disease. These symptoms were consistent with those of bacterial canker described by Vidaver & Davis (1988) and were found only on cv. Target. A slow-growing, Gram-positive, pale-yellow bacterium was consistently isolated on yeast dextrose carbonate agar medium (Lelliot & Stead, 1987). Sixteen representative isolates were identified as Clavibacter michiganensis ssp. michiganensis (similarity 47–89%) using fatty acid methyl ester (FAME) analysis and Sherlock Microbial Identification System software (Microbial ID, Newark, DE, USA) (Gitaitis & Beaver, 1990). Pathogenicity was tested by spraying 5-week-old-tomato seedlings (cv. Easy Harvest) with bacteria suspended (108 cfu mL−1) in sterile water. Control plants were sprayed with sterile water. After inoculation plants were covered with polyethylene bags for 48 h and maintained in a greenhouse at 25°C. Symptoms similar to those observed in the field developed on the inoculated plants within 7–13 days. No symptoms developed on control plants. The bacterium was re-isolated from inoculated plants and its identity confirmed by FAME analysis. Although it has been reported previously in Aegean and Mediterranean regions (Özaktan, 1991), this is the first record of bacterial canker on tomatoes in the eastern Anatolia region of Turkey. Occurrence of the disease only on plants of cv. Target suggests that contaminated seeds and/or transplants of cv. Target may have been the source of the pathogen.

Effects of Snowfall Fluctuation on Tree Growth and Establishment of Subalpine Abies mariesii near Upper Forest-Limit of Mt. Yumori, Northern Japan

We examined effects of annual snowfall fluctuation on tree growth and establishment in a mature stand (ca. 200 yr old) of Japanese subalpine Abies mariesii Mast, near upper forest-limits at leeward site. Relatively tall and aged fir trees (>5 m, >100 yr old) retained various scars of past mechanical damage due to snow pressures, e.g., multiple-, tip dieback- and broken-stems, and canopy anomaly. Ring-width pattern analysis showed that these deformed trees mostly experienced abrupt growth reduction. Some events of such abrupt growth reduction occurred simultaneously just after winters with heavy snowfall (e.g., mid-1960s), indicating that the A. mariesii trees often suffered from snow damage due to settlement force of excess snowpack. Particularly, in snowy winters, mechanical breakage of branches was likely to occur intensively at the height of 1 to 3 m above the maximum snowpack level of normal snowfall winters (ca. 3 m). On the other hand, some aged, but suppressed, trees sharply enhanced their growth rates simultaneously after such period of the intensive snow damage. This indicated that the snow damage upon the relatively tall trees sometimes facilitated regeneration of nearby shaded-trees by creating small-scale gaps. The snow-induced disturbance events may have occurred at about 10-yr intervals during the last half of 20th century. Our findings suggested that population dynamics of A. mariesii near its upper forest-limit, especially located at leeward and snowy site, was closely affected by the repetitive events of snow damage following snowy winters: the event primarily acted as a major limiting factor for canopy development of individuals, and sometimes triggered growth of suppressed-trees.

Effects of Snowfall Fluctuation on Tree Growth and Establishment of Subalpine Abies mariesii near Upper Forest-limit of Mt. Yumori, Northern Japan

We examined effects of annual snowfall fluctuation on tree growth and establishment in a mature stand (ca. 200 yr old) of Japanese subalpine Abies mariesii Mast, near upper forest-limits at leeward site. Relatively tall and aged fir trees (>5 m, >100 yr old) retained various scars of past mechanical damage due to snow pressures, e.g., multiple-, tip dieback- and broken-stems, and canopy anomaly. Ring-width pattern analysis showed that these deformed trees mostly experienced abrupt growth reduction. Some events of such abrupt growth reduction occurred simultaneously just after winters with heavy snowfall (e.g., mid-1960s), indicating that the A. mariesii trees often suffered from snow damage due to settlement force of excess snowpack. Particularly, in snowy winters, mechanical breakage of branches was likely to occur intensively at the height of 1 to 3 m above the maximum snowpack level of normal snowfall winters (ca. 3 m). On the other hand, some aged, but suppressed, trees sharply enhanced their growth rates simultaneously after such period of the intensive snow damage. This indicated that the snow damage upon the relatively tall trees sometimes facilitated regeneration of nearby shaded-trees by creating small-scale gaps. The snow-induced disturbance events may have occurred at about 10-yr intervals during the last half of 20th century. Our findings suggested that population dynamics of A. mariesii near its upper forest-limit, especially located at leeward and snowy site, was closely affected by the repetitive events of snow damage following snowy winters: the event primarily acted as a major limiting factor for canopy development of individuals, and sometimes triggered growth of suppressed-trees.

Seedling Blights of Cyperaceae Weeds Caused by Curvularia tuberculata and C. oryzae

Two Curvularia tuberculata isolates (93-020 and 93-022) and one C. oryzae isolate (93-061) were obtained from diseased Cyperus difformis , C. iria , and Fimbristylis miliacea , respectively, in the Philippines in 1993. When inoculated onto their hosts at 1 2 10 8 conidia m -2 , and provided with 24-h dew at 28°C, a severe and rapid blighting of the leaves caused the weed seedlings to die within two weeks. The three Curvularia isolates were cross-pathogenic to C. difformis , C. iria , F. miliacea , and C. rotundus but exhibited varying degrees of virulence depending on the weed species being attacked. C. tuberculata isolates 93-020 and 93-022 effectively controlled C. difformis and C. iria but were less effective on F. miliacea. C. oryzae was equally virulent on C. difformis , C. iria , and F. miliacea . C. rotundus was not killed by any of the three isolates and exhibited only minor flecking and leaf tip die-back. Sporulation of the three Curvularia isolates on necrotic leaf tissues occurred on the susceptible ( C. difformis , C. iria , and F.miliacea ) but not on the resistant ( C. rotundus ) weed hosts.

Cryptodiaporthe Canker of Pagoda Dogwood in North Dakota.

Cryptodiaporthe canker, caused by Cryptodiaporthe corni (Wehm.) Petrak (anamorph Myxosporium nitidum Berk. & Curt.), is known to occur only on pagoda dogwood (Cornus alternifolia L. fil.) throughout its native range in eastern Canada and most of the eastern United States (1). Cankered branches with yellow-to-orange pigmented bark were recently observed on four ≈25-year-old C. alternifolia plants in an ornamental planting on the campus of North Dakota State University in Fargo. Although the fungal anamorph was usually present on all symptomatic materials, the teleomorph was also present on branches with larger diameters (>15 mm). Collections of infected branches made on 25 April 2001 contained perithecia but did not contain mature ascospores. Shoot growth began during the first week of May. Subsequent collections made on 11 May and 8 June 2001 contained perithecia with mature ascospores. A previous collection made on 12 October 2000 contained fully developed perithecia devoid of ascospores. A range of symptoms was present among the individual trees: one tree was severely cankered (up to 4 cm diameter) with many dead branches; one tree showed only minor damage with tip dieback on a few small twigs; and two trees had intermediate symptoms (1 cm diameter with cankers) with several dead branches. Pagoda dogwood is generally seed propagated, and differences in disease severity observed in these plants may be the result of genetic variation in resistance. Selection for resistant genotypes may be a possibility. Specimens (BPI 840955A and B) consisting of cankered branches containing both fungal morphs were deposited at the U.S. National Fungus Collections at Beltsville, MD. Although Cryptodiaporthe canker was previously collected from a native woodland tree in Minnesota (1), the current North Dakota specimens were collected from cultivated plants in a landscape setting. To our knowledge, this is the first report of Cryptodiaporthe canker in North Dakota. Management of this disease may need to be considered when recommending C. alternifolia for horticultural plantings. Reference: (1) S. C. Redlin and A. Y. Rossman. Mycologia 83:200, 1991.

Xanthomonas cassavae . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Xanthomonas cassavae . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Manihot esculenta ( Euphorbiaceae ); by artificial inoculation: Euphorbia pulcherrima ( Euphorbiaceae ). DISEASE: Cassava leaf spot or bacterial necrosis. Angular leaf spots extend along veins but generally do not develop into blight. Spots age, turn dark brown and are surrounded by a yellow halo. Exudate is frequently produced. On stems, dark green point lesions develop slowly up to 1 cm diam. Lytic pockets generally develop under lesions in the cortex. Lateral extension can lead to girdling and tip dieback. Secondary colonization by Colletotrichum gloeosporioides [Glomerella cingulata] is frequently observed. Systemic infection and vascular browning are absent or very restricted. Entry into the host is through natural openings (stomata) or epidermal wounds, which can be caused (especially on the stem) by sand particles or small grains of gravel thrown up by the strong winds which precede the first rains. The optimum temperature for disease development is 25°C. Disease mainly occurs above altitudes of 800 m. There is evidence that disease severity is linked to poor plant nutrition. GEOGRAPHICAL DISTRIBUTION: AFRICA: Burundi, Congo Democratic Republic, Kenya, Malawi, Niger, Rwanda, Tanzania, Uganda, Zaire. SOUTH AMERICA: Colombia. TRANSMISSION: Long distance spread is restricted. Symptomless cuttings taken from diseased plants were unable to demonstrate propagation. Rapid disease development under favourable climatic conditions suggests a symptomless epiphytic phase on the host itself or on a plant other than cassava. In the field, dispersal is by wind and rain.

First report of a phytoplasma associated with a disease of date palms in North Africa

White tip die-back (WTD) is a newly recognized disease of young date palms (Phoenix dactylifera L.) in northern Sudan and occurs in scattered foci throughout the region. It affects immature palms, 5–8 years old, which die within 6–12 months of symptom appearance. Symptoms appear as a severe chlorosis of the emerging spear leaf and at the tips of the pinnae of older fronds. Chlorotic streaks, mainly white with some necrosis, extend longitudinally along the sides of the frond mid-rib. There is no yellowing of the crown, which changes quickly from green to dry white. No phytoplasmas could be unequivocally identified by electron microscopy in the WTD-affected date palm tissues. Using a nested polymerase chain reaction together with general phytoplasma primers which amplify the 16S/23S rDNA (Cronjéet al., 1998), phytoplasma-specific products, 1250 bp in size, were consistently amplified from WTD palms but not from healthy date palms. The phytoplasma 16S/23S rDNA intergenic spacer region was sequenced and this showed a very high (99%) homology with comparable sequences of Bermuda grass white leaf (BGWL) phytoplasma strains from South-east Asia and Sudan (GenBank accession numbers: Y14645, AF100412). Further studies will be needed to establish if there is a connection between this date palm disease and the presence of BGWL in plantations. The 16S/23S spacer sequence from WTD date palm has been submitted to GenBank (accession number: AF 100411). Date palms are known to be susceptible to the lethal yellowing phytoplasma in the USA (Harrison et al., 1995), but there are no previous records for their susceptibility to phytoplasma infection in countries outside the lethal yellowing zone.

Slow decline: a new disease of mature date palms in North Africa associated with a phytoplasma

Slow Decline (SD) or El Arkish is an established lethal disease of uncertain aetiology that affects mature date palms (Phoenix dactylifera L.) in Northern Sudan. It is common along the Nile between Dongola and Merowe–Karema, where it causes steady annual losses estimated at 6%. Palm mortality occurs between 12 and 24 months after the first appearance of symptoms. SD symptoms include a yellowing of the lower canopy starting with the outermost (oldest) fronds and progressing towards the young central fronds and emerging spear leaf. All fronds dry white to light-brown and are then shed, leaving an erect dry tuft of young leaves at the top of the trunk which may break off to leave the trunk standing alone. In some case, the white dry spear leaves on palms can be seen from the ground prior to the appearance of any foliar yellowing. Young offshoots of SD-affected palms also show frond yellowing and white spear necrosis as the crown dies. Occasionally, the spear can be pulled out to reveal rotten, foul-smelling basal tissues. Although mature palms were too tall to observe any inflorescence or fruit symptoms, there was no evidence of fruit drop. Using a nested polymerase chain reaction together with general phytoplasma primers which amplify the 16S/23S rDNA (Cronje et al., 1998), phytoplasma-specific products, 1250-bp in size, were amplified from four of six SD-affected palms (one trunk and three of five petiole samples) but not from symptomless palms from the same area. The sequence of the SD phytoplasma 16S/23S rDNA intergenic spacer (GenBank Accession number AF268000) showed a very high (99%) homology with comparable sequences of phytoplasmas associated with White tip die-back, a newly reported disease of young date palms (Cronjéet al. 2000), and Bermuda grass white leaf (BGWL) disease from Sudan (Genbank Accession numbers AF100411, AF100412).

Bacterial Blight of Welsh Onion : A New Disease Caused by Xanthomonas campestris pv. allii pv. nov.

In June of 1998, a new bacterial disease was observed on Welsh onion in Okinawa Prefecture, Japan. Infected plants in nursery boxes were stunted with tip dieback, and heavily infected plants died. In fields, the disease appeared on leaves as irregular gray spots or elliptical spots with creases in the center. These spots enlarged and spread rapidly continued cloudy or rainy weather, and formed blight lesions on outer leaves. Yellow mucoid bacterial colonies were consistently isolated from these lesions. The causal bacterium was identified as a pathovar of Xanthomonas campestris on the basis of bacteriological properties. The bacterium was pathogenic to Welsh onion, onion, but nonpathogenic to chive, Chinese chive and hyacinth. Of Liliaceae plants, which contain Welsh onion and onion, only hyacinth has been reported as a host for the genus Xanthomonas, namely X. campestris pv. hyacinthi. However, strains of X. campestris pv. hyacinthi were not pathogenic against either Welsh onion or onion. From these results, the bacterium isolated from Welsh onion is considered to be a new pathovar of X. campestris, and the name of X. campestris pv. allii pv. nov. is proposed. A strain MAFF 311173 is designated as the pathotype strain.

First Report of a Leaf Blight of Onion Caused by Xanthomonas campestris in Colorado.

Sweet Spanish onion (Allium cepa L.) cultivars in southern Colorado (Otero and Prowers counties) have been found with symptoms of a foliar blight since 1996, and the same symptoms have been observed in northern Colorado (Weld County) since 1997. This onion disease appears to be identical to that reported from Barbados in 1971, Hawaii in 1978, and Texas in 1998 (1). Leaf blighting in scattered fields began as linear, tan to brown, water-soaked lesions that rapidly coalesced and were often surrounded by chlorotic areas, causing a general discoloration and tip die-back of affected foliage. The disease occurs generally after periods of heavy rainfall or storms. Bulb size may be reduced, and 10 to 15% yield losses have been recorded from control plots in copper-based bactericide screening nurseries naturally infected primarily by this pathogen at Rocky Ford during 1996 to 1998. Disease progression into bulbs has not been observed. Gram negative, rod-shaped, yellow bacteria were consistently recovered from infected foliar and bulb tissues on nutrient agar. The bacterial isolates utilized glucose in an oxidative manner, were catalase positive, oxidase negative, and negative for the tetrazolium salt tolerance test (0.1 and 0.02%). Two strains, one recovered during 1996 and the other in 1997, were identified by Microbe Inotech Laboratories (St. Louis, MO) as Xanthomonas campestris based on fatty acid methyl ester analysis (similarity indices of 0.81 and 0.82). A literature search indicated that classification to pathovar is lacking (2). To prove pathogenicity, a suspension (108 CFU/ml sdw) of one of the strains was sprayed to runoff onto a flat of approximately 100 8-week-old plants. Inoculated plants were placed in a dew chamber for 24 h, and then transferred to a bench and maintained at 25 to 28°C with a 12-h photoperiod and misting period for 14 days. A sample of 10 randomly selected plants with symptoms of water-soaking and discoloration was collected from which X. campestris was reisolated. No symptoms developed on seedlings sprayed with water only. This is the first report of X. campestris from onions grown in Colorado and the western continental United States.

Pathogenicity and Pyrenocine Production of Curvularia inaequalis Isolated from Zoysia Grass.

Five isolates of Curvularia inaequalis were obtained from blighted leaves of zoysia grass. The optimal growth temperature of the pathogen was 30°C and all isolates caused foliar blighting symptoms on zoysia grass, bent grass, and bermuda grass. Phytotoxic substances were associated with pathogenicity. Two phytotoxins were isolated from liquid cultures of C. inaequalis by ethyl acetate extraction and repeated silica gel column chromatography. On the basis of mass and nuclear magnetic resonance spectral analyses, the compounds were identified as pyrenocines A and B, phytotoxins produced by Pyrenochaeta terrestris. Pyrenocine A caused leaf necrosis in a leaf-wounding bioassay and inhibited growth of all plants tested in a whole plant test. Large crab grass and fall panicum were most susceptible to pyrenocine A. Pyrenocines A and B also caused significant electrolyte leakage from leaf tissues of bermuda grass. Pyrenocine B exhibited much weaker phytotoxic activity than pyrenocine A in all bioassays performed. Both compounds caused leaf tip dieback symptoms in turf grass plants similar to symptoms observed in the field. Thus, pyrenocines A and B are thought to be involved in the development of Curvularia blight disease of turfgrasses caused by C. inaequalis.

623 Lowbush Blueberry Response to Phosphorus Fertilizers with Different P: N Ratios

In three commercial fields with a history of low leaf P concentrations, triple super phosphate (TSP) (1 P: 0 N), monoammonium phosphate (MAP) (2.1 P: 1 N), and diammonium phosphate (DAP) (1.11 P: 1 N) with P at 67.2 kg·ha-1 were compared to a control in a randomized complete-block design with 12 blocks. In 1995, all fertilizer treatments were comparable in raising soil P concentrations, but MAP and DAP resulted in higher P leaf concentrations compared to the control. DAP was more effective than MAP in raising N leaf concentrations. Leaf concentrations of Mg, B, and Cu were lowered by MAP and DAP but not TSP. Stem density, stem length, flower buds per stem, flower bud density, and yield were raised by DAP. The same treatments were applied in May 1997 and in May 1999 to the same plots in the same fields. In 1997, by the time of tip dieback in the prune year of that cycle, foliar concentration of P and N averaged higher than in the previous cycle, but still were not up to the standard for N. Fruit yield for the second cycle averaged substantially higher for the controls and for all three treatments, most dramatically for the DAP. In 1999, with only two fields available, response to treatments depended on soil N availability. At the field where leaf N was lower in control plots, MAP and DAP were more effective than TSP in raising leaf P.

Seedling Creeping Bentgrass (Agrostis stolonifera) Tolerance to Dithiopyr

Dithiopyr was applied to ‘Penneagle’ creeping bentgrass seedlings 2, 4, and 8 wk after emergence (WAE) in separate studies in the summers of 1992 and 1993 to determine seedling tolerance. Dithiopyr emulsifiable concentrate (IEC) was applied at 0.14, 0.28, 0.42, and 0.56 kg ai/ha, and granular (G) dithiopyr (0.1G) was applied at 0.14, 0.21, 0.28, and 0.42 kg/ha. Dithiopyr applied 2 WAE caused unacceptable injury regardless of rate or formulation. Injury was characterized by stunting of plants, leaf tip die-back, yellowing of older leaves, and a visible decline in quality. Injury increased with increasing rate. In 1992, the EC and G formulation of dithiopyr caused similar responses, whereas in the summer of 1993, the EC caused greater turf injury. Dithiopyr applied 4 WAE caused less injury than when applied 2 WAE, and injury was acceptable. Dithiopyr applied 8 WAE only slightly injured creeping bentgrass at the high rate for both formulations in both years.