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Rubbery taproot disease on sugar beet in Serbia

A ProMED-mail post
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Date: Tue 6 Oct 2020
Source: Plant Disease [edited]
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[Ref: Z Curcic et al (2020): Rubbery Taproot Disease of Sugar Beet in Serbia Associated with '_Candidatus_ Phytoplasma solani'. Plant Disease 104, published online; DOI: 10.1094/PDIS-07-20-1602-RE]

Rubbery taproot disease (RTD) of sugar beet was observed in Serbia for the 1st time in the 1960s. The disease was already described in neighbouring Bulgaria and Romania at the time, but it was associated with abiotic factors. In this study on sugar beet in Serbia, we provide evidence of the association between _Ca._ P. solani (stolbur phytoplasma) infection and the occurrence of typical RTD symptomatology.

_Ca._ P. solani [CPa] was identified by PCR and sequence analyses of 16S rRNA, _tuf_, _secY_ and _stamp_ genes. The agent of the syndrome "basses richesses" [SBR] of sugar beet, _Ca._ Arsenophonus phytopathogenicus [CAp], was not detected in any of the 134 symptomatic and asymptomatic sugar beets collected. [This], at the very least, excluded [it] as the main etiological agent, if present at all. [PCR] analyses confirmed _Ca._ A. phytopathogenicus in positive controls from SBR-affected sugar beet from Germany. [Thus,] RTD in Serbia was associated with [CPa].

[CPa] has been occasionally detected in SBR-affected sugar beets in France besides [CAp], but not in Germany and Switzerland and was not found to play a significant role in SBR epidemics. The symptoms of RTD in Serbia are strikingly different from those reported in France [for SBR]. Symptoms of RTD caused by [CPa] are closer to those of sugar beet yellow wilt caused by another phytoplasma (16SrIII-J) [X-disease group] in South America.

Sequence analysis of the stolbur _tuf_ gene confirmed a previously reported and a new, distinct genotype (named 'tuf d') that is prevalent in sugar beet. The sequence signature of the _tuf_ gene as well as the one of _stamp_ both correlate with the epidemiological cycle and reservoir plant host.

This enables the differentiation of stolbur strains associated with RTD from closely related strains, thereby providing information to identify insect vectors and reservoir plant hosts. The results indicate that there are differences in [beet] hybrid susceptibility.

Clarifying the etiology of RTD as a long-known and economically important disease is the first step towards disease management. Other diseases of unknown etiology with similar symptoms in other parts of the world should be reconsidered.

Communicated by: ProMED-mail <promed@promedmail.org>

[Syndrome des Basses Richesses (SBR; ProMED-mail post
https://url.emailprotection.link/?beWM3nO2hG1deX_BPaNEUsDcBwRb3p04HbHCkgZnPRzdhZYDOiapkDVWCkJ1DduVuZY4pXheyFyrY_-qs3R4f1vF6HcNQptw3owQgRJKtCBdLwlZyFn1p1dYQBEzTeoq3) was 1st described on sugar beet (_Beta vulgaris_) crops in the Burgundy and Franche-Comte regions of France. Symptoms on sugar beet include yellowing and curling of older leaves leading to new growth of central leaves which show chlorosis and distortion. Roots are often distorted but of normal size, vascular bundles are brown. Sugar content of the beets suddenly decreases before harvest in autumn. Up to 50 percent loss of income from a crop, due to both reduced sugar content and reduced plant vigour, have been reported.

Two phloem-restricted prokaryotes which cannot be cultured, a phytoplasma (Stolbur SBR phytoplasma, 16SrXII taxonomic group) and at least one gamma-3 proteobacterium (SBR proteobacterium), have been found associated with the disease. Phylogenetic analysis showed a close relationship between the SBR proteobacterium and several bacteria in the arsenophonus clade, which contains secondary symbionts of hemipteran insects. Two tentative names are currently in use for the proteobacterium, _Ca._ Phlomobacter betae and _Ca._ Arsenophonus phytopathogenicus. The relationship of these pathogens to _Ca._ Phlomobacter/Arsenophonus fragariae associated with marginal chlorosis of strawberry in France and Japan remains to be clarified. More work is needed to clarify the epidemiology of these recently identified pathogens as well as the taxonomic relationships of species within the arsenophonus clade. Until taxonomy can ultimately resolved, the possibility can also not be excluded that there may be more than one species in the clade that is associated with diseases of sugar beet in different areas.

Both the SBR phytoplasma and proteobacterium are transmitted by plant hopper vectors, in particular _Pentastiridius leporinus_. Disease management for any of these pathogens may include vector control, removal of pathogen and/or vector reservoirs (volunteer crop plants, weeds), use of clean planting material and crop rotation with non-host species. Phytosanitary measures for prevention of the pathogens to spread to new areas would be vital.

Stolbur phytoplasmas, alone or in coinfections, have been reported to affect a range of dicotyledoneous crops (including tomato, potato, capsicum, aubergine, grapevine, sugar beet), as well as a few monocot hosts such as maize and sugarcane (ProMED-mail posts https://url.emailprotection.link/?byTc-CUg82h1Qm_OEBW9feoSBv3GukfA_gNjyRZZTlnMkJJFOua2jE0dWqqmvRS0sIO0V29fiXeM8IqSLMqN6GnPNU85HihJUyRG-dAhOh1RWeoQh7LQN9_b8kQvKxQ0D and https://url.emailprotection.link/?bMliY0oF8GwdVwHQ4en3LKieTg6RcgZNK6PLn7QVixhJavcvQsE34EtqgKs_Bg5GnNJtHqUyfUdT5IRd54-DtM1P_5ne8RZkDnPLTWOo_rbbgXObKvusLXgXrM1CCRYWh).

Maps

Serbia (with districts):

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Europe, overview:

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Pictures

SBR symptoms:

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(affected field),

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(tuber, healthy on left),