Introduction and significance
This disease is an important cause of yield loss in some production areas, notably New Zealand, USA, and parts of Europe. In other areas, disease severity varies and is dependent on wet weather in the latter part of the growing season. Yield is reduced when purple spot causes premature defoliation and diseased, unmarket- able spears.
Symptoms and diagnostic features
Two types of purple spot inoculum, ascospores and conidia, cause similar symptoms. Small, elliptical lesions that measure 2–6 mm in diameter develop on the emerging spears. Lesions are often sunken and brown with purple margins. These infection sites are shallow and do not penetrate deeply into the spear. However, the prominent lesions still reduce market- ability of the product. Severe attacks result in large numbers of lesions. Damage to spears from wind- blown soil increases the degree of infection. On mature fern, the symptoms are small white lesions with red- brown margins that are on individual leaflets and stems (128). Disease causes yellowing and defoliation of the needles (129). Loss of the foliage can weaken the crown and reduce yields in subsequent seasons.
Causal agent
The cause of purple spot is the ascomycete fungus
Pleospora herbarum. Spherical perithecia mature over
several months and produce yellow-brown ascospores that have seven transverse septa and measure 38 x 18 μm. The anamorph of this pathogen is Stemphylium
vesicarium. Conidia of S. vesicarium have verrucose
walls, are yellow-brown to olive-brown, measure 25–42 x 12–22 μm, and have one to six (usually three) transeverse septa and one to three longitudinal septa. The spores have a conspicuous basal scar. Conidia are borne on mainly unbranched, yellow-brown to olive- brown conidiophores that have a distinctly swollen apical cell with a pore.
DISEASES OFVEGETABLECROPS
F
UNGAL
D
ISEASES
128 Stem lesions of purple spot of asparagus.
128
129 Yellowing of asparagus leaves caused by
Stemphylium.
135
Disease cycle
The pathogen survives on crop residues and fern debris, and produces perithecia that release ascospores in the spring. Spear infection is associated with periods of wet weather during the harvest period. Symptoms may appear on spears in less than 24 hours after a rain. Infection occurs through stomata, and long periods (greater than 16 hours) of surface wetness are required for severe infection to develop. This pathogen does not persist in the soil.
Control
Remove and dispose of fern residues prior to emergence of spears in spring. Differences in cultivar susceptibility may exist, so select more tolerant cultivars. Short compact plants with low branches (for example, the culivar Cito) are generally more susceptible than tall erect types (cv. Jersey King). Apply protectant fungi- cides, such as chlorothalonil, to the fern.
References
Evans, T. A. and Stephens, C. T. 1984. First report in Michigan of the teleomorph of Stemphylium vesicarium, causal agent of purple spot of asparagus. Plant Disease 68:1099.
Falloon, P. G., Falloon, L. M., and Grogan, R. G. 1987. Etiology and epidemiology of Stemphylium leaf spot and purple spot of asparagus in California. Phytopathology 77:407–413. Hausbeck, M. K., Hartwell, J., and Byrne, J. M. 1997.
Epidemiology of Stemphylium leaf spot and purple spot in no-till asparagus. Proceedings of IX International Asparagus
Conference, 48–53.
Johnson, D. A. 1990. Effect of crop debris management on severity of Stemphylium purple spot of asparagus. Plant
Disease 74:413–415.
Johnson, D. A. and Lunden, J. D. 1986. Effects of wounding and wetting duration on infection of asparagus by Stemphylium
vesicarium. Plant Disease 70:419–420.
Lacy, M. L. 1982. Purple spot: a new disease of young asparagus spears caused by Stemphylium vesicarium. Plant Disease 66:1198–1200.
Meyer, M. P., Hausbeck, M. K., and Podolsky, R. 2000. Optimal fungicide management of purple spot of asparagus and impact of yield. Plant Disease 84:525–530.
Suberi, H. and Price, T. V. 2000. Infection of leaves by Alternaria
porri and Stemphylium vesicarium and disease development
in controlled environments. Plant Pathology 49:375–382. Sutherland, P. W., Hallet, I. C., Parkes, S. L., and Templeton,
M. D. 1990. Structural studies of asparagus spear infection by
Stemphylium. Canadian Journal of Botany 68:1311–1319.
Puccinia asparagi
RUST
Introduction and significance
Rust occurs worldwide wherever asparagus is grown. The disease is an important cause of early defoliation, and hence yield loss for subsequent years, in production areas in North America and Europe. In the UK, rust was of minor significance for many years, but it remains a threat as indicated by serious outbreaks in 1997.
Symptoms and diagnostic features
After asparagus harvest is completed, subsequent shoots are allowed to develop and grow into the foliage or fern. Rust disease infects this fern foliage and causes dark to brown-red pustules on the stems and needles (130). Rust can spread rapidly, causing extensive ASPARAGUSOFFICINALIS F UNGAL D ISEASES 130 Stem pustules of rust of asparagus. 130
136
infection of the foliage (131). Later in the season, the pustules may become black as the teliospores are formed (132). Severe infection results in weakened plants and reduced yield.
Causal agent
Rust disease is caused by the basidiomycete fungus
Puccinia asparagi, an autoecious, macrocyclic rust. The
fungus forms uredinia pustules that release brown, one- celled urediniospores that measure 19–30 x 18–25 μm. Later in disease development, the same pustules will form black, two-celled teliospores that measure 30–50 x 19–26 μm. Two other spore types, spermagonia and aecidiospores, can also develop on yellowish lesions in the spring but are often overlooked.
Disease cycle
Like most rust fungi, the life cycle is complex. Disease is often initiated by teliospores that germinate to produce intermediate spore types that eventually form aecidia. Aecidiospores penetrate the asparagus host and result in uredinia and urediniospores during the summer. Teliospores are produced in increasing numbers towards the end of the season. Sporulation is optimal at 25–30º C. Spore germination requires surface moisture for only 3 hours (optimum 9 hours) at 15º C. The com- bination of warm days and cool nights with dew formation is particularly favorable for rust develop- ment.
Control
Remove and dispose of fern residues prior to emergence of spears in spring. Remove volunteer asparagus plants that might be infected. Apply fungicides such as triazole products. Resistant cultivars are not yet available.
References
Beraha, L., Linn, M. B., and Anderson, H. W. 1960. Development of asparagus rust pathogen in relation to temperature and moisture. Plant Disease Reporter 44:82–86. Blanchette, B. L., Groth, J. V., and Waters, L. 1982. Evaluation of asparagus for resistance to Puccinia asparagi. Plant Disease 66:904–906.
Fantino, M. G., Granier, A., Solaini, J., and Di Carmine, D. 1990. Four years of trials (1985–1988) on asparagus rust (Puccinia
asparagi D. C.) in Lazio. Acta Horticulturae 271:371–375.
Johnson, D. A. 1986. Two components of slow-rusting in asparagus infected with Puccinia asparagi. Phytopathology 76: 08–211.
Johnson, D. A. and Lunden, J. D. 1992. Effect of rust on yield of susceptible and resistant asparagus cultivars. Plant Disease 76:84–86.
Mullen, R. J. and Viss, T. C. 1996. Control of asparagus rust in the Sacramento-San Joaquin delta region of California.
Proceedings of the VIII International Symposium on Asparagus. Acta Horticulturae 415:297–300
.
DISEASES OFVEGETABLECROPS
F UNGAL D ISEASES 132 Black teliospores of rust of asparagus. 132
131 Dieback of asparagus foliage infected with rust.
137
Asparagus virus 1, 2, 3,Tobacco streak virus