Introduction and significance
This is a major root rot disease of alliums, with onion and garlic being particularly affected. White rot can also be a common problem in garden settings. In some regions white rot is an increasing problem due to intense crop rotations and shortage of uninfested land. In the USA and other regions, another Sclerotium species, S. rolfsii, causes southern blight disease on alliums (36).
Symptoms and diagnostic features
White rot affects roots and crowns, and overall symptoms are usually first noticed only after root infection is well established. Foliage of infected plants turns yellow, wilts, collapses, and eventually dies and becomes brown and dry (36, 37). In badly affected areas, foliage growth is poor and patches of plants die rapidly. White, persistent mycelium develops on diseased roots and the base of bulbs in contact with soil. Numerous tiny, black, spherical sclerotia, measuring less than 1 mm in diameter, form on mycelium and diseased tissue (38). In advanced stages of disease, the roots and bulbs become soft and rotted due to activity from secondary decay organisms. Symptoms on leek are usually less severe than on onion or garlic.
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36 Onion infected with southern blight.
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Causal agent
The cause of white rot is Sclerotium cepivorum, which has no known perfect stage. Conidia are likewise not produced, though small spermatia occur on germinat- ing hyphae but appear to have no role in disease devel- opment. Sclerotium cepivorum reproduces, survives, and infects by sclerotia (39) that consist of a smooth black rind that is two to five cell layers deep and an inner medulla of closely packed hyphae. The pathogen can be readily isolated on solid media such as potato dextrose agar. The white rot fungus is host specific to allium crops. The other species, S. rolfsii, causes a watery rot of alliums and produces brown, spherical sclerotia that are usually significantly larger (1–2 mm in diameter) than those of S. cepivorum. Sclerotium rolfsii has a very broad host range.
Disease cycle
White rot is associated with soilborne inoculum, so affected areas and disease incidence increase as alliums are cropped in infested fields. Sclerotia can persist in soil without a plant host for over 20 years. These propagules remain dormant until allium root exudates, such as propyl and alyl cysteine amino acids that are specific to this group of plants, are present in soil. Soil microorganisms metabolize the amino acids into the stimulatory compounds alkyl and alkenyl thiols and
sulphides. Sclerotia germinate and the resulting mycelium grows 1–2 cm through soil and invades the host root and basal plate. Secondary spread can occur by mycelial growth from plant to plant if their roots are in close proximity. Temperature is a key factor in disease development as sclerotia show little activity below 9º C or above 24º C; the optimum range is 14–18º C. Crops planted in the autumn may therefore
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37 Onion field affected by white rot.
37 38 Garlic infected
with white rot.
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39 Sclerotia and mycelium of white rot on onion.
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experience reduced disease severity. Sclerotia germinate under moist conditions (not below –100 kPa [ = –1bar]), but germination is inhibited in very wet soils. Bulb onion crops established from transplants are more severely affected than direct seeded crops because of more vigorous root systems that develop earlier in the season and hence encounter more soilborne inoculum.
Control
Crop rotation is of limited value because sclerotia persist in soil for many years. Rather, select fields and plant crops where there is no history of the disease. Implement sanitation measures to prevent introduction of infested soil and contaminated equipment into clean fields. Use only disease-free transplants. Be aware that sclerotia survive passage through the digestive tracts of grazing animals and may therefore be present in manures. Truly resistant cultivars are not yet available. Soil fumigation with dazomet, metam sodium, methyl bromide, and chloropicrin on heavily infested soils can be useful treatments, though the fungus is not eradicated. Fungicides used as onion seed or garlic clove treatments or transplant drenches can be partially effective. Drenches with dicarboximide fungicides were initially useful, but enhanced microbial degradation of these compounds in some soils led to a decline in their per- formance. Triazole fungicides are now used for white rot control in some countries.
Soil inoculum can be reduced if sclerotia can be stim- ulated to germinate in the absence of host plants. Such germination is triggered by diallyl disulphide (DADS), which mimics the stimulatory activity of allium roots and is now commercially available in some countries. Effectiveness of this treatment relies on thorough appli- cation to soil and appropriate temperature and moisture conditions. Note that not all sclerotia respond to this treatment. Composted onion waste is also showing promise for white rot control in the UK when incorporated back into the field. Composting tempera- tures must reach 50º C to kill any sclerotia present in onion waste.
Though not completely effective, flooding of infested fields is used in some regions to manage white rot. This is most effective when temperatures are above 20º C. Solarization reduced sclerotial populations in Australia and Egypt and might form part of an integrated control strategy where climatic conditions are suitable.
References
Brix, H. D. and Zinkernagel, V. 1992. Screening for resistance of
Allium species to Sclerotium cepivorum with special reference
to non-stimulatory responses. Plant Pathology 41:308–316. Coley Smith, J .R. 1986. Interactions between Sclerotium
cepivorum and cultivars of onion, leek, garlic and Allium fistulosum. Plant Pathology 35:362–369.
Coley Smith, J. R. 1990. British Society for Plant Pathology Presidential Address 1989. White rot disease of Allium: problems of soil-borne disease in microcosm. Plant Pathology 39:214–222
Coley Smith, J. R. and Entwistle, A. R. 1988. Susceptibility of garlic to Sclerotium cepivorum. Plant Pathology 37:261–264. Couch, B. C. and Kohn, L. M. 2000. Clonal spread of Sclerotium
cepivorum in onion production with evidence of past
recombination events. Phytopathology 90:514–521. Crowe, F. J .and Hall, D. H. 1980. Vertical distribution of
sclerotia of Sclerotium cepivorum and host root systems relative to white rot of onion and garlic. Phytopathology 70:70–73.
Entwistle, A. R. 1986. Controlling allium white rot (Sclerotium
cepivorum) without chemicals. Phytoparasitica 20:121–125.
Entwistle, A. R. 1992. Loss of control of Allium white rot by fungicides and its implications. Aspects of Applied Biology 12:201–209.
Esler, G. and Coley Smith, J. R. 1983. Flavour and odour characteristics of species of Allium in relation to their capacity to stimulate germination of sclerotia of Sclerotium
cepivorum. Plant Pathology 32 :13–22.
Gladders, P., Wafford, J. D., and Davies, J. M. L. 1987. Control of allium white rot in module raised bulb onions. In: T. J. Martin (ed.). Application to Seeds and Soil. BCPC
Monograph No. 39:371–378.
Koike, S. T., Gonzales, T. G., and Oakes, E. D. 1994. Crown and root rot of chives in California caused by Sclerotium rolfsii.
Plant Disease 78:208.
Leggett, M. E. and Rahe, J. E. 1985. Factors affecting the survival of sclerotia of Sclerotium cepivorum in the Fraser Valley of British Columbia. Annals of Applied Biology 106:255–263. Melero-Vara, J. M., Prados-Ligero, A. M., and Basallotte-Ureba,
M. J. 2000. Comparison of physical, chemical and biological methods of controlling garlic white rot. European Journal of
Plant Pathology 106:581–588.
Smolinska, U. 2000. Survival of Sclerotium cepivorum sclerotia and Fusarium oxysporum chlamydospores in soil amended with cruciferous residues. Journal of Phytopathology 148:343–349.
Smolinska, U. and Horbowicz, M. 1999. Fungicidal activity of volatiles from selected cruciferous plants against resting propagules of soilborne fungal pathogens. Journal of
Phytopathology 147:119–124. DISEASES OFVEGETABLECROPS
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Urocystis cepulae (= U. colchici var. cepulae)
SMUT
Introduction and significance
Smut occurs in many onion growing areas and is occa- sionally important. It affects bulb and salad onion, leek, shallot, and chives. Garlic appears to be immune and resistance is present in Allium fistulosum.
Symptoms and diagnostic features
Symptoms can appear as early as seedling emergence, when cotyledons show infections. Cotyledons, true leaves, and leaf sheaths develop oblong to elongated, dark, raised blisters on outer surfaces (40). These growths can cause downward curling of the leaves. Mature blisters break open to expose black, powdery fungal growth (41). There is progressive spread of the disease inwards, which can kill seedlings within 3 to 4 weeks. Infected bulb tissue remains firm, though secondary decay organisms can penetrate damaged areas and cause rot. Another disease, smudge (caused by Colletotrichum circinans), also produces black fungal growth on onion, leek, and shallot, so differenti- ating smut from smudge in the field may be difficult.
Causal agent
Smut is caused by Urocystis cepulae (synonymous with
Urocystis colchici var. cepulae). It is a basidiomycete
fungus belonging to the Ustilaginales. This pathogen produces distinctive black spore masses that consist of chlamydospores. These are spherical, single-celled, brown to black, 12–15 μm in diameter, with an outer layer of small sterile cells, 46 μm in diameter.
Disease cycle
The pathogen is soilborne and can survive for up to 20 years in soil. The pathogen may be introduced on sets or transplants. Seedborne infection is not considered important. Chlamydospores can be spread by winds and water, and optimum temperatures for germination are 13–22º C. Most plant infection occurs at 10–12º C and disease activity is greatly reduced above 25º C.
Control
Use fungicide-treated seed and resistant cultivars if available. Use healthy transplants because these are able to resist infection from soilborne inoculum. Plant when soil temperatures are higher.
References
Utkhede, R. S. and Rahe, J. E. 1980. Screening world onion germplasm collection and commercial cultivars for resistance to smut. Canadian Journal of Plant Science 60:157–161.
ALLIACEAE F UNGAL D ISEASES 41 Black sporula- tion of smut on green onion. 41 40 Smut infection on green onion. 40
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Garlic yellow stripe virus, Garlic yellow streak virus, Leek yellow stripe virus