Introduction and significance
This disease occurs wherever table beets and Swiss chard are grown and is one of the most important diseases affecting plants in the chenopodium group. The disease results in significant damage in warm temperate areas, including southern Europe, the Mediterranean region, Japan, Russia, and the USA. Crop losses can exceed 40% on a root weight basis and, in extreme cases, almost complete crop loss can occur. The disease is particularly problematic when the crop is grown for its foliage.
Symptoms and diagnostic features
Circular leaf spots are small initially, up to 2 mm in diameter, and can be very numerous. Spots have a pale brown or off-white center with a reddish margin (135); the latter is useful for distinguishing it from Ramularia leaf spot. Spots expand in size, remain circular or oblong, and can result in extensive loss of foliage. The centers of the spots become gray as the fungus produces dark mycelium and other structures within and on the leaf tissue.
Causal agent
Cercospora leaf spot is caused by the fungus
Cercospora beticola. The conidia are borne on
unbranched brown conidiophores growing in clusters from stomata. Conidia are hyaline, with three to 14 septa, and are long and thread-like in shape. Conidia range in length from 78–228 μm, and are slightly tapered from base (4.4–6.3 μm width) to tip (1.6–3.2 μm width). Spore length and septation are influenced by environmental conditions; spores may be up to 400 μm long and have up to 27 septa. In leaf tissue the fungus makes dark mycelial clumps that are called sclerotia. A perfect stage has not been reported. Hosts include sugar beet, table beet, Swiss chard, wild sea beet, spinach, and various Atriplex and Chenopodium weeds. The pathogen is seedborne. On sugar beet, various physio- logical races have been identified.
Disease cycle
Spores are splash and wind dispersed and may be carried in irrigation water. Sporulation, germination, and infection occur most rapidly at 25–35º C when night temperatures are above 16º C and relative humidity is above 90%. There is limited disease activity below 15º C. Leaf penetration occurs only via stomata. Dry periods, of up to 6 hours during the day, enhance infection compared with continuous wetness. Spots appear in 7 to 10 days after infection under optimal conditions. Cercospora beticola overwinters as sclerotia in infected leaves and can survive in soil for up to 2 years. Initial inoculum comes from infested seed, weed hosts, and spores produced on infected crop residues.
Control
Use resistant cultivars if available. For sugar beet, specific and non-specific resistance has been identified. Bury infected crop residues and destroy volunteer plants. Do not plant vegetable production crops close to seed crops. Use seed that does not have significant levels of the pathogen. For infested seed, apply seed treatments. Apply fungicides prior to infection and symptom development. Isolates resistant to sterol demethylation-inhibiting (DMI) fungicides have been documented, so use products with different modes of action to reduce the risks of further resistance problems. In the UK, strobilurin plus triazole fungicide combina- tions have recently been approved for use against
Cercospora on sugar beet. Disease prediction systems
are used in some countries to better time fungicide applications.
DISEASES OFVEGETABLECROPS
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ISEASES
135 Cercospora leaf spot on beet.
141
References
Calpouzos, L. and Stallknecht, G. F. 1967. Symptoms of Cercospora leaf spot of sugar beets influenced by light intensity. Phytopathology 57:799–800.
Karaoglanidis, G. S. and Thanassoulopoulos, C. C. 2003. Cross- resistance patterns among sterol biosynthesis inhibiting fungicides (SBIs) in Cercospora beticola. European Journal of
Plant Pathology 109:929–934.
Karaoglanidis, G. S., Ioannidis, P. M., and Thanassoulopoulos, C. C. 2002. Changes in sensitivity of Cercospora beticola populations to sterol-demethylation-inhibiting fungicides during a 4-year period in northern Greece. Plant Pathology 51:55–62.
Moretti, M., Arnold, A., D’Agostini, A., and Farina, G. 2003. Characterization of field-isolates and derived DMI-resistant strains of Cercospora beticola. Mycological Research 107:1178–1188.
Pundhir, V. S. and Mukhopadhyay, A. N. 1987. Epidemiological studies on Cercospora leaf spot of sugar beet. Plant Pathology 36:185–191.
Wallin, J. R. and Loonan, D. V. 1971. Effect of leaf wetness duration and air temperature on Cercospora beticola infection of sugarbeet. Phytopathology 61:546–549.
Erysiphe betae
POWDERY MILDEW
Introduction and significance
This common disease occurs in Europe, the Middle East, and parts of the USA. It affects sugar beet, fodder beet, table beet, and Swiss chard, as well as wild sea beet (Beta maritima). Severe attacks can reduce yield by up to 25% and table beet crops with badly diseased foliage may be unmarketable.
Symptoms and diagnostic features
The first signs of powdery mildew are scattered, small, radiating colonies consisting of superficial white fungal growth. Such growth tends to start on the older leaves and occurs on both upper and lower leaf surfaces. As the disease develops, the entire leaf can become heavily colonized with extensive patches of the white powdery fungus (136, 137). Older leaves with severe powdery mildew will turn yellow and can later senesce and die. Younger leaves can also be affected if disease in the field is severe.
Causal agent
Powdery mildew is caused by the fungus Erysiphe
betae. This pathogen is a typical powdery mildew
fungus having both sexual and asexual stages on beet and related hosts. Asexual conidia are produced basipetally in short chains arising from mycelium on the leaf surface. Conidia are barrel-shaped, hyaline, and variable in size depending upon environmental condi- tions. Conidia are smaller (measuring 40 x 14 μm) when temperatures and relative humidity are low and larger (measuring 49 x 18 μm) when both factors are high. The sexual stage consists of a spherical cleisto- thecium, that is initially yellow, later turns dark brown to black, and measures 0.1 mm in diameter. Cleisto- thecia have numerous simple or branched hyphal appendages and contain four to six asci, each ascus bearing three to five ascospores. Ascospores measure 20–30 x 14–16 μm. BETA VULGARIS F UNGAL D ISEASES
136 Beet leaf showing powdery mildew.
136
137 Beet severely affected by powdery mildew.
142
Disease cycle
Powdery mildew conidia have a high water content (60%) that enables these spores to germinate at low relative humidity. Germination occurs over a wide range of environmental conditions, with an optimum of 25º C and 70–100% relative humidity. Spores penetrate the leaf within a few hours and then produce hyphal growth and chains of conidia. As conidia are dislodged from the leaf and dispersed by winds, a new spore will mature at the basipetal end of the conidial chain. Spores are short-lived but may be transported in high altitude winds over long distances. Epidemics are associated with dry weather alternating with periods of high relative humidity and temperatures above 20º C. Plant susceptibility increases with age, and damage is more severe on drought-stressed plants because of leaf loss. Frequent rains slow the progress of the disease. Powdery mildew overwinters in buried roots (in Europe these are called groundkeepers) and on volunteer beet and weed hosts. Cleistothecia can be found on crop debris, which is another means of overwintering. Cleistothecia release ascospores during rain.
Control
Powdery mildew control relies on fungicides. Apply treatments at the onset of symptoms. Sulfur fungicides and newer triazole products have been effective. After harvest, plow down and destroy crop residues, and remove volunteer plants. If possible, do not plant vegetable beet in close proximity to sugar beet fields. A disease forecasting system in the UK, based on the number of ground frosts in February and March, helps guide control strategies in sugar beet. These forecasts could be useful to vegetable growers, as they provide an indication of disease risk and inoculum pressure from sugar beet crops.
References
Asher, M. J. C. and Williams, G. E. 1991. Forecasting the national incidence of sugar beet powdery mildew from weather data in Britain. Plant Pathology 40:10–107. Hills, F. J., Chiarappa, L., and Geng, S. 1980. Powdery mildew of
sugar beet: Disease and crop loss assessment. Phytopathology 70:680–682.
Ruppel, E. G., Hills, F. J., and Mumford, D. L. 1975. Epidemiological observations on the sugarbeet powdery mildew epiphytotic in western U. S. A. in 1974. Plant Disease
Reporter 59:283–286.
Peronospora farinosa f. sp. betae
DOWNY MILDEW
Introduction and significance
This downy mildew affects sugar beet, mangold, Swiss chard, wild sea beet, and table beet. Other forma speciales (f. sp.) affect different members of the Chenopodiaceae; for example, P. farinosa f. sp.
spinaciae is the important downy mildew of spinach.
Beet downy mildew is found in almost all beet producing areas and is most common in mild climates such as coastal California and Oregon in the USA and in parts of Europe. Downy mildew has been associated with malformed roots of table beet in the UK. When downy mildew causes death of the apical growing point, plant growth is severely hampered and losses of more than 50% can result.
Symptoms and diagnostic feature
Downy mildew can infect plants at any stage, and initial symptoms are chlorosis and distortion of the youngest leaves. The pathogen grows systemically within the young leaves and may invade the growing point, resulting in reduced growth and a rosette of spindly, deformed, chlorotic leaves, which curl downwards (138). Such leaves can be noticeably thickened and brittle. Under humid conditions, a dense, purple-gray growth appears on both upper and lower leaf surfaces. Individual downy mildew lesions can also develop on leaves. These lesions are chlorotic, irregular in shape, and later dry and turn brown. If the growing point has been invaded, a dark heart rot of the crown may develop. This heart rot can be difficult to recognize as a downy mildew symptom if the infected leaves have senesced and dropped off the plant. If older plants are infected, such plants can recover and produce addi- tional leaves that are healthy. In seed crops, the main seed stalk is stunted and distorted, sepals and bracts are swollen, and the overall effect is a ‘witch’s broom’ symptom.
DISEASES OFVEGETABLECROPS
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Causal agent
Downy mildew is caused by the oomycete P. farinosa f. sp. betae. The pathogen produces dichotomously branched sporangiophores that emerge through stomata. The sporangia are produced at the tips of the sporangiophore branches, are hyaline to pale violet, and measure 20–28 x 17–24 μm. Oospores measure 26–36 μm and under cool moist conditions are produced in infected vegetative tissues and seed. The pathogen was previously known as P. schachtii.
Disease cycle
The pathogen can be seedborne and therefore spread from place to place in seed. About 1% of infested seeds give rise to infected plants. Oospores in the soil germinate to produce mycelium and then sporangia. Overwintering mycelia in seed crops, weed hosts, and volunteer plants also produce sporangia. Sporangia are airborne and are carried by wind to host plants. For severe epidemics to develop, cool, moist conditions (optimum temperature is 8º C) are required. Sporangia form at 5–22º C and 60–100% relative humidity (optimum 12º C, 85% relative humidity) and germinate at 1–30º C (optimum 4–10º C). At least 6 hours of leaf wetness and cool temperatures (optimum 7–15º C) are required for infection. There is little infection at tem- peratures above 20º C.
Control
Use seed that does not have significant levels of the pathogen. Use seed that has been produced in dry regions that discourage downy mildew development. Remove volunteer and weed plants that harbor the pathogen. Destroy crop residues after harvest. There is some disease resistance available in sugar beet. Use fungicides as required; protectant sprays of dithiocar- bamates or phenylamides have given some control.
References
Byford, W. J. 1981. Downy mildews of beet and spinach. In: The
Downy Mildews (Ed. D. M. Spooner) pp. 531–543.
Academic Press London.
MacFarlane, J. W. 1968. Elimination of downy mildew as a major sugarbeet disease in the coastal valleys of California.
Plant Disease Reporter 52:297–299.
Pleospora bjoerlingii (anamorph = Phoma betae)