Survival and Persistence of Alternaria dauci in Carrot Cropping Systems.

Alternaria dauci was recovered in California from carrot crop residue and from volunteer carrot plants in fallow carrot fields. The fungus was not recovered from common weeds surrounding fallow fields. To evaluate further the survival of A. dauci on carrot crop residue, infected carrot leaf tissue was placed in fields or in soil in greenhouse pots, and recovered over time. In California, A. dauci was recovered from infected leaf tissue in both fallow and irrigated fields for as long as 1 year. In Florida, A. dauci was recovered from infected leaf tissue in fallow fields for up to 30 weeks. In greenhouse experiments, A. dauci was recovered from infected leaf tissue for as long as 1 year in dry soil, but only up to 30 weeks in soil that was watered weekly. To determine the infectivity of A. dauci borne on carrot crop residue, infected carrot crops were incorporated into organic and mineral field soils, and soil samples were collected over time. Carrot seed were planted in collected soil, and seedling infection by A. dauci was recorded. Seedling infection was detected up to 13 and 14 weeks after crop incorporation in organic and mineral soil, respectively. Seedling infection was detected for up to 5 weeks in soil that remained dry compared with 3 weeks in flooded soil.

A New Disease of Flowering Dogwood Caused by Colletotrichum acutatum.

In 1992 symptoms consisting of twig dieback, canker-like deformities, and often, mortality of all sizes of container-grown flowering dogwood trees (Cornus florida L.) was frequently observed in northern Florida where nursery production of dogwood is concentrated. Twigs, branches, growing points, and portions of main trunks up to 3 cm in diameter were killed. On affected portions, leaves drooped downward but did not wilt severely; they later became silvery, gray-green and eventually died but did not drop. Affected trees produced flower buds, but the flower buds did not open. Many trees died and others were destroyed because of unsightly dead limbs and deformed trunks. The inciting pathogen was not satisfactorily determined. After 1994, disease incidence subsided, but in 1999, it reappeared and caused serious economic damage. In 1999, we followed the disease from propagation through several stages of nursery production, including growth in large containers. A Colletotrichum species was consistently isolated from acervuli produced in abundance near the soil line on diseased, rooted cuttings and small plants. The same fungus was consistently isolated from acervuli produced on diseased leaves and twigs of larger plants and small trees. Conidia of the isolated pathogen were used to inoculate small dogwood trees kept at 100% RH for 2 days then grown in a greenhouse. Within 23 weeks, a slowly developing leaf spot was produced on inoculated plants. After 3 months, numerous acervuli were produced on inoculated leaves and on adjacent small twigs from which the same pathogen was consistently reisolated. The symptoms and pathogen were not consistent with descriptions of anthracnose incited by Discula destructiva. On potato-dextrose agar (PDA), spore masses were bright red-orange on lawn plates inoculated with large numbers of conidia. Conidiomata did not produce setae in culture nor did acervuli on diseased plant material. On PDA, mycelial growth was tufted and pale-gray. The reverse side of colonies was buff to cream-colored or pale-gray to tan but never dark. With age, a pale pink or orange-pink pigment often formed within the agar media (1,3). When produced on PDA, most conidia were elliptical and elongated with abruptly tapering ends. An average of 18% had both ends slightly rounded; none had only one end abruptly tapered or rounded. Conidia measured 15.3 × 4.78 µm; the length/width ratio was 3.20 (1,3). The teleomorph was not found on diseased plant material or in culture. Analysis of DNA by polymerase chain reaction (PCR) using the CaInt2-ITS4 and CgInt-ITS4 primer pairs (1,2) and by comparison of PCR products with those produced by C. acutatum pathotypes isolated from leatherleaf fern, lime, post-bloom fruit drop-affected Navel orange fruit, and strawberry fruit, and with isolates of C. gleosporiodes from citrus, Camellia, Nandina, holly, and strawberry indicated that the fungus was C. acutatum. The amplified PCR product (approximately 490 bp) obtained from all dogwood isolates using primers CaInt2 and ITS4 was consistent with the size of product expected from C. acutatum. No products were produced with the CgInt-ITS4 primer pair. Based on the morphology of conidia, growth in culture and PCR results, the pathogen was identified as C. acutatum and represents the first report of this pathogen on flowering dogwood. References: (1) J. E. Adaskaveg and R. J. Hartin. Phytopathology 87:979, 1997. (2) A. E. Brown et al. Phytopathology 86:523, 1996. (3) B. C. Sutton. Pages 1-27 in: Colletotrichum: Biology, pathology and control. Brit. Soc. For Plant Pathol., CAB International, 1992.

Asymmetric infectivity of pseudorecombinants of cabbage leaf curl virus and squash leaf curl virus: implications for bipartite geminivirus evolution and movement.

The bipartite geminiviruses squash leaf curl virus (SqLCV) and cabbage leaf curl virus (CLCV) have distinct host ranges. SqLCV infects a broad range of plants within the Cucurbitaceae, including pumpkin and squash, and CLCV has a broad host range within Brassicaceae that includes cabbage and Arabidopsis thaliana. Despite this, the genomic A components of these viruses share a high degree of sequence identity, particularly in the gene encoding the replication protein AL1, and their common regions are 77% identical. However, there is unexpected sequence diversity in the common regions of the two CLCV genomic A and B components, these being only 80% identical. Based on these sequence similarities, we investigated the host range properties of pseudorecombinants of SqLCV and CLCV. We found that in a pseudorecombinant virus consisting of the A component of CLCV and the B component of SqLCV, both components replicated in tobacco protoplasts, and this pseudorecombinant was infectious and caused systemic disease in Nicotiana benthamiana, a common host to all bipartite geminiviruses. However, this pseudorecombinant did not move systemically in pumpkin or Arabidopsis, despite the demonstrated replication compatibility of the genome components. As a result of the greater sequence differences between the common regions, the pseudorecombinant of SqLCV A and CLCV B components neither replicated the CLCV B component nor systemically infected any of the hosts tested. These findings demonstrate that for different geminiviruses with distinct host ranges, the replication origins and AL1 proteins can be sufficiently similar to permit infectious pseudorecombinants, but replication alone is not sufficient to cause systemic disease, and host range may ultimately be limited at the level of movement. The results of this study further suggest that CLCV is an evolving virus that can provide insights into how new bipartite geminiviruses arise from mixed infections.

Survival of Colletotrichum acutatum in Soil and Plant Debris of Leatherleaf Fern.

Leatherleaf fern anthracnose is incited by a new pathotype of Colletotrichum acutatum. Fern anthracnose appeared in Florida in 1993, has spread rapidly, and produces losses that approach 100% in some ferneries. To help develop recommendations for control and management of fern anthracnose, the survival of the pathogen in soil and diseased plant material and on clothing was investigated in laboratory and field experiments. Survival of conidia, sclerotia, and conidia in infected leaf debris was studied in soil maintained at five soil moisture levels. Survival of conidia and sclerotia declined rapidly under moist conditions (≥12% moisture, vol/wt), but under dry conditions, viable conidia could be detected up to 12 months after incorporation into soil. Similar results were obtained in field experiments in which infected leaf debris was buried in soil. Viable conidia were recovered for up to 3 months in leaf debris stored under laboratory conditions. Conidia applied to denim fabric were recovered up to 5 weeks after application. This indicated that contaminated clothing could transport conidia effectively within and between ferneries. Inoculating ferns with dilutions of conidia demonstrated that at least 100 conidia per ml is required to infect and colonize leatherleaf fern leaves. This threshold provides an estimate of levels of conidia populations likely to result in disease and levels that sanitation or cultural practices must exceed to be effective management methods.