ALGUNOS COMENTARIOS COMPLEMENTARIOS RELATIVOS AL GASTO MUNICIPAL POR SECTORES, PROGRAMAS Y SERVICIOS

Introduction and significance

Carrot motley dwarf (CMD) is a sometimes damaging disease of carrots grown in temperate regions. The disease occurs primarily in the USA (California) but is also common in the UK. CMD also affects parsley, cilantro, and dill.

Symptoms and diagnostic features

When young carrot plants are affected, growth is stunted and there is pronounced reddening, chlorotic mottling, and overall yellowing of the foliage (95). There may be twisting of the lower leaves. Roots are severely stunted (96) and some plants may die. Young parsley plants show reddening of the older leaves and a general yellowing of the foliage, and plants may become stunted (97). Infection of older carrot and parsley plants produces milder symptoms and plants may be symp- tomless when temperatures are above 24º C. In general, leaf discoloration of all Apiaceae plants may be caused by a variety of factors, so virus tests are required to confirm CMD disease.

Causal agents

CMD disease is caused by a co-infection of two viruses:

Carrot red leaf virus (CRLV), which is a polerovirus

with isometric particles (25 nm in diameter), and Carrot

mottle virus (CMoV), an umbravirus having single

DISEASES OFVEGETABLECROPS

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95 _{Carrot field affected with carrot motley dwarf.}

117 stranded RNA but lacking a protein coat. Both viruses

can infect plants individually, but CMD disease is only produced when both viruses are present. When present as single viruses, only CRLV can be transmitted by the aphid vector. CMoV cannot be vectored by the aphid unless this virus is accompanied by the helper virus CRLV; the CRLV proteins encapsidate the RNA of CMoV, enabling transmission by aphids. The mixture of viruses is transmitted by the willow carrot aphid (Cavariella aegopodii) in a persistent manner. CMD is restricted to cultivated and wild Apiaceae, and the viruses are not seedborne.

Disease cycle

For carrot, disease spreads when these viruses are vectored between successive or nearby carrot crops. Presence of the aphid vector and CMD disease in over- wintered and volunteer carrots is an important source of the problem for newly planted carrots. Wild Apiaceae, such as wild carrot (Daucus carota) and cow parsley (Anthriscus sylvestris), are also potential sources of CMD viruses.

Control

In some regions, control has been achieved by planting spring crops at least 1.5 km away from overwintered crops. In California, reducing the number of carrot fields that are kept intact through the winter has signif- icantly reduced disease occurrence in new, spring planted carrots. Remove volunteer and weed hosts before new crops emerge. Carrot cultivars vary in their response to CMD, so use more tolerant cultivars if available.

References

Morton, A., Spence, N. J., Boonham, N., and Barbara, D. J. 2003. Carrot red leaf associated RNA in carrots in the United Kingdom. Plant Pathology 52:795.

Waterhouse, P. M. and Murant, A. F. 1983. Further evidence on the nature of the dependence of carrot mottle virus on carrot red leaf virus for transmission by aphids. Annals of Applied

Biology 103:455–464.

Watson, M. T. and Falk, B. W. 1994. Ecological and epidemiological factors affecting carrot motley dwarf development in carrots grown in the Salinas Valley of California. Plant Disease 78:477–481.

Watson, M. T., Tian, T., Estabrook, E., and Falk, B. W. 1998. A small RNA resembling the beet western yellows luteovirus ST9-associated RNA is a component of the California carrot motley dwarf complex. Phytopathology 88:64–170.

APIACEAE

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96 Stunted carrots infected with carrot motley dwarf.

Healthy plant is on the right.

96

97 _{Carrot motley dwarf causing yellowing and}

reddening of parsley foliage.

118

Erysiphe heraclei

POWDERY MILDEW

Introduction and significance

Powdery mildew is probably the most important foliar disease of parsnip in the UK. Powdery mildew is more damaging to parsnip than to carrot or celery. Early attacks can cause significant yield losses.

Symptoms and diagnostic features

The symptoms usually appear first on the petioles and leaf blades of older foliage. The superficial white fungal growth is similar to that of other powdery mildews. In the UK, little infection is seen until crop canopies close over in July. Younger leaves become infected as disease progresses. When powdery mildew is severe, both upper and lower leaf surfaces are affected and develop yellowing and early loss of the foliage (98, 99).

Causal agent

Powdery mildew is caused by the ascomycete Erysiphe

heraclei. For a description of this pathogen, see the

carrot section on powdery mildew in this chapter (page 103). There may be some host specialization, but isolates appear to be able to infect a number of Apiaceae host species.

Disease cycle

Overlapping parsnip plantings, volunteer parsnip, and weed hosts are potential sources of infection. The disease cycle can be completed in 7 days under optimum conditions. High humidity and moderate temperatures favor infection. Crops become more sus- ceptible with age. Disease severity is increased by drought stress and is reduced by rain or overhead irrigation.

Control

Apply fungicides such as sulfur or triazole products if severe disease is a possibility. Such materials are strictly protectants and must be applied prior to significant disease development. Avoid over-fertilizing because high nitrogen rates can result in succulent foliage that is very susceptible to powdery mildew.

References

Palti, J. 1975. Erysiphaceae affecting Apiaceae crops, with special reference to carrot, in Israel. Phytopathologia Mediterranea 14:87–93.

Itersonilia pastinacae, I. perplexans

ITERSONILIA CANKER,