Diseases of Vegetable, Ornamental and Spice Crops (2+1)

VIRAL DISEASES

4. RHIZOMANIA
Symptoms:

• Leaves may show flabbiness, wilting and mild yellowing on the infected sugarbeet plant in the field.
• The most characteristic symptoms found in beet leaves are a pattern of yellow areas along the veins.
• However, the virus mostly remains confined to roots and rarely moves to the foliage.
• Symptoms on roots are characterized by root stunting and a proliferation of lateral rootlets on the main tap root, giving it bearded appearance.

Pathogen:

• The virus responsible for this disease is Beet Necrosis Yellow Vein Virus (BNYVV) which is a probable member of the furovirus (fungus borne rod shaped virus) group.
• Ultrathin sections of BNYVV- infected tissue show the virus particles scattered in the cytoplasm and in angled layer aggregates.
• The virus is not seen in large aggregates but shows an erratic distribution in the mesophyll cells of systemic plants.

Disease cycle and epidemiology:

• The fungus Polymyxa betae has been considered to be the vector of BNYVV and was also shown to transmit soil borne virus.
• This fungus has worldwide distribution.
• The abnormal proliferation of the root lets, characteristic of rhizomania was found in the presence of non-viruliferous P. betae but zoospores transmit rhizomania symptoms to whole sugar beet plants, in the absence of detectable virus in the infected plants, indicating that BNYVV is not requisite to the etiology of the disease.
• The infectivity of BNYVV was retained with P. betae in air and dry soil for at least 15 years.
• BNYVV was detected by ELISA in extracts of resting spore clusters of a viruliferous P. betae isolate.

Management:

• Varieties Laetitia and Nagano have been reported to possess resistance to this disease. Soil disinfection with dicloropropene was recommended for the control of fungal vector P. betae.
• Preplant application of fumigants like dichloropropene, tolone 11 (1,3- dichloropropene), Vorled (1,3-dichloropropene + methyl isothiocyanate), Vorlex 201 (1,3- dichloropropene + chloropicrin + methyl isothiocyanate) and Pichlor 60 reduced disease incidence and significantly increased yields. Soil inoculation with T. harzianum reduced infection by vector, P. betae.