3.- FUENTES DE FINANCIACION
V.- EVOLUCION DEL GASTO PUBLICO EN SERVICIOS SOCIALES
Introduction and significance
Phloeospora leaf spot is generally a minor leaf disease of parsnip. In the UK severe infection usually occurs on late summer and autumn parsnip crops. A Phloeospora disease also has been reported from New Zealand.
Symptoms and diagnostic features
The first symptoms are small (1–2 mm) pale green or brown leaf spots. As disease develops, leaf spots increase in number, spots coalesce to form large necrotic blotches (2–5 cm), and infected leaf tissue becomes gray brown (101). The disease spreads rapidly through the crop, causing extensive leaf death and defoliation (102). No root symptoms have been reported. Severely affected leaf tissue has tiny black fruiting bodies called acervuli. Spore tendrils ooze from acervuli and form characteristic white patches on leaf surfaces (103).
Causal agent
Phloeospora leaf spot is caused by the fungus
Phloeospora herclei. Fruiting bodies are minute, cup-
shaped structures called acervuli. Conidia are formed in these structures and are curved, one to four septate (though most are single septate), and measure 50–80 x 3.5–5 μm. The early literature probably confused
P. herclei with Ramularia pastinaceae. The Phloeospora
leaf spot disease reported from New Zealand is attrib- uted to P. crescentium. However, P. herclei and P. cres-
centium may be the same organism.
Disease cycle
Little is known about the epidemiology of this pathogen. Inoculum is spread by splashing water and direct contact between leaves. Phloeospora herclei probably survives on infected debris and on weed hosts such as hogweed (Heracleum sphondylium).
Control
Specific control measures have not been developed.
References
Laundon, G. F. 1970. Records of fungal plant diseases in New Zealand. New Zealand Journal of Botany 8:51–66. Riley, E. A. 1952. Leaf spot of parsnip caused by Phloeospora
crescentium (Barth.) n. comb. Mycologia 44:213–215. DISEASES OFVEGETABLECROPS
F
UNGAL
D
ISEASES
101 Necrotic leaf blotches of Phloeospora leaf spot of
parsnip.
101
103 White spore masses of Phloeospora leaf spot of
parsnip.
103
102 General necrosis of parsnip foliage caused by
Phloeospora leaf spot.
121
Causal agent
Phoma canker is caused by Phoma complanata. Colonies are light to olivaceous gray, have even margins, and produce dense aerial mycelium. Pycnidia measure 35–49 μm in diameter and have thick walls. Conidia are hyaline, guttulate, and variable in shape. Conidia dimensions are 5–9 x 2–3.5 μm (mean 7.4 x 2.4 μm), though large (27 x 8 μm) one-septate spores occur in older cultures or lesions.
A second pathogen, Phomopsis diachenii, can be confused with the Phoma canker organism. Reddish brown leaf spots with deep purple margins measure 10–20 mm in diameter and contain black pycnidia. Microscopic examination shows that P. diachenii, like most Phomopsis species, produces both filiform and oval spores.
Disease cycle
Leaf spots and petiole lesions of Phoma canker appear 4 to 5 days after infection, and disease rapidly pro- gresses if favorable conditions are present. Insects are reported to contribute to secondary spread. Root infection occurs when spores are washed down from the infected foliage. In the UK, foliar symptoms may be absent even though root cankers are present; in these situations, the source of infection is likely to be soilborne inoculum. Phoma complanata is seedborne and can reduce seedling establishment. Diseased roots contain elevated levels of furocoumarins. These sub- stances are photocarcinogens, so take precautions to prevent skin contact when handling parsnip roots.
APIACEAE F UNGAL D ISEASES Phoma complanata
PHOMA CANKER
Introduction and significance
This disease has increased in importance since it was first reported in Canada in 1984. The pathogen is host specific to parsnip and related weeds, such as cow parsnip (Heracleum lanatum). Field diagnosis of Phoma canker is difficult because several pathogens produce similar symptoms and more than one causal agent may be present on diseased plants. In the UK, P.
complanata may now be the most important cause of
parsnip root cankers.
Symptoms and diagnostic features
If present, leaf spots are circular, brown, and often have yellow halos. As disease develops, leaf spots coalesce and result in a general yellowing, necrosis, and blighting of foliage. Pycnidia appear in leaf lesions about two weeks after infection. Dark brown to black lesions may develop on petioles, resulting in leaves that twist and bend over. Large, dark brown to black cankers form on parsnip crowns and on the sides of taproots (104). These cankers can enlarge and affect much or all of the crown tissue. Cankers result in splitting or cracking on the upper portion of the root. Careful examination of the surfaces of cankers will reveal pycnidia embedded in these tissues (105). Pycnidia ooze a white or creamy spore exudate. Affected roots are reported to have a sweet cinnamon odor.
104 Root symptoms of Phoma canker of parsnip.
104
105 Decayed parsnip root with pycnidia of the Phoma
canker pathogen.
122
Control
Rotate crops so that a 4-year break occurs between parsnip plantings. Use seed that does not have signifi- cant levels of the pathogen. Choose and plant cultivars that are known to have tolerance or reduced suscepti- bility to this pathogen. Monitor and sample crops regularly so that harvest can be scheduled early if Phoma canker starts to appear. Application of fungi- cides may help control foliar symptoms; however, chemicals are not effective at preventing disease caused by soilborne inoculum.
References
Cerkauskas, R. F. 1985. Canker of parsnip caused by Phoma
complanata. Canadian Journal of Plant Pathology
7:135–138.
Cerkauskas, R. F. 1987. Phoma canker severity and yield loss in parsnip. Canadian Journal of Plant Pathology 9:311–318. Cerkauskas, R. F. 1987. Phoma canker of parsnips. Ministry of
Agriculture Fisheries and Food, Ontario, Canada. Leaflet 87- 041, 3pp.
Cerkauskas, R. F. and Mc Garvey, B. D. 1988. Fungicidal control of Phoma canker of parsnips. Canadian Journal of Plant
Pathology 10:252–258.
Plasmopara umbelliferarum, (P. crustosa/P. nivea)
DOWNY MILDEW
Introduction and significance
Downy mildew occurs on parsnip, but in the UK severe crop damage is rare. It also occurs on carrot, celery, chervil, fennel, and parsley. Downy mildew isolates affecting carrot are reported only in Europe.
Symptoms and diagnostic features
Symptoms are yellow, angular shaped leaf blotches. The lesions soon turn brown and leaves develop a ragged appearance as the centers of lesions fall out. White sporulating growth develops on the undersides of the lesions (106). Young, succulent growth is most suscep- tible to infection and leaves with multiple lesions turn yellow and senesce (107).
Causal agent
Downy mildew is caused by an oomycete in the genus
Plasmopara. This produces monopodial branched
sporangiophores that emerge from leaf stomates. Smaller branches that bear sporangia are arranged at right angles to the supporting branches. Sporangia are ovoid, hyaline, have a single pore on the distal ends, and measure 23–27 x 17–19 μm. Oospores are produced within infected tissues and seeds. The nomenclature of the various downy mildews that infect Apiaceae is under review. Some researchers believe the names
P. crustosa and P. nivea are not valid, and propose that P. umbelliferarum be used as the pathogen name.
DISEASES OFVEGETABLECROPS
F
UNGAL
D
ISEASES
106 Parsnip leaf infected with downy mildew.
106
107 Necrotic leaf tissue of parsnip caused by downy
mildew.
123
Disease cycle
Infection is favored by cool, wet conditions. Free moisture is required for sporangia to germinate and release zoospores through the distal pore. These swimming spores then invade the leaf via stomata.
Control
Control measures are not usually required. Rotate to non-host crops to reduce the risk of infection from soilborne oospores. When symptoms appear early in the season, apply fungicides.
References
Constantinescu, O. 1990. The nomenclature of Plasmopara parasitic on Umbelliferae. Mycotaxon 43:471-477.
Ramularia pastinacae