ProMED
http://www.promedmail.org

Source: India Today [summ., edited]
https://www.indiatoday.in/india-today-insight/story/why-kerala-has-a-coconut-problem-1838024-2021-08-10

Coconut cultivation has been in decline in India. The national production comes mainly from Kerala, Tamil Nadu, Karnataka, and Andhra Pradesh. A major problem is the threat from root wilt disease. Agricultural scientists have so far failed to find a solution to the problem. Coconut farmers and industry stakeholders say serious intervention is required, especially in Kerala. The Coconut Development Board said that developing root wilt resistant varieties is the key. The 1st root wilt case was detected in 1920 in Kerala's Kottayam district. But even after 100 years, no solution has been found to control the disease.

[Byline: Jeemon Jacob]
--
Communicated by:
ProMED
<promed@promedmail.org>

[Kerala wilt (KW, also called root wilt) is spreading in parts of Kerala and adjacent states. It is considered the most serious problem for coconut cultivation in the affected areas. KW is caused by a phytoplasma which is currently considered unclassified, after initially having been included in the lethal yellowing group (see below). In addition, another phytoplasma of the 16SrXI group (_Ca._ P. oryzae) has also been reported from KW affected palms. Further research is needed to clarify the role of the 2 phytoplasmas in KW.

A range of yellowing diseases of coconut and other palms caused by phytoplasmas are known. Lethal yellowing (LY) diseases have been described from the Caribbean, Latin America, Africa, and southern Asia caused by strains of _Candidatus_ Phytoplasma palmae (16SrIV). Cape St Paul wilt (CSPW) in Ghana, the "maladie de Kaincope" in Togo, Bogia wilt in Papua New Guinea, and Awka disease (lethal decline) in Nigeria have been classified in the new 16SrXXII group (Nigerian coconut lethal decline group, _Ca._ P. palmicola strains). The phytoplasma causing Weligama wilt in Sri Lanka is as yet unclassified. Some of the pathogens may also affect herbaceous crops, such as banana or cassava (e.g. ProMED posts 20180207.5613931 and 20170713.5168553).

In contrast to some of these diseases which can kill the host within a few months, KW is not lethal but significantly reduces production. Symptoms may include drooping leaflets; yellowing and marginal necrosis of older leaves; necrosis of inflorescences; as well as root rot. Fungal leaf rot is often found as a secondary infection. The leaf hopper _Proutista moesta_ has been identified as a potential vector. It has been suggested that weeds as pathogen reservoirs may play a role in the epidemiology of KW (and other LY diseases).

The palm phytoplasmas are spread by insect vectors, such as plant hoppers, but further research on epidemiology is needed. Different vectors may be involved in the spread of strains in different areas. Jumps of LY across apparently unaffected coconut populations have been observed, possibly due to aerial spread of infectious vector insects or human activities. Seed transmission of the pathogens cannot be excluded. Disease management relies on eradication of affected palms, improving the general conditions of coconut groves, as well as replanting with resistant or tolerant varieties if available (e.g. ProMED post 20210618.8456768). Symptoms can be suppressed by tetracycline treatments, usually applied as trunk injections. The antibiotic inhibits multiplication of the pathogens but does not eliminate them. Therefore, treatments need to be repeated regularly.

For KW, resistance breeding programmes based on disease escapees from heavily KW affected areas have been in progress for some years (ProMED post 20150902.3614963), but apparently have not yet resulted in varieties suitable for farmers. At least for LY phytoplasmas, susceptibility may vary between coconut cultivars or even within cultivars, depending on the region where they grow; an unexplained resistance breakdown of some widely used hybrids (ProMED post 20070522.1643) is causing great concern.

Maps
India (with states):
https://www.worldometers.info/img/maps/india_physical_map.gif and
http://healthmap.org/promed/p/308
Kerala districts:
https://www.mapsofindia.com/maps/kerala/kerala.htm

Pictures
Kerala wilt symptoms:
https://4.bp.blogspot.com/_zTn4rB23H8Q/SPC-20xyhhI/AAAAAAAAA0o/HpXtCt8_r5s/s320/b2.jpg and
http://thumbs.dreamstime.com/z/coconut-tree-root-wilt-disease-top-affected-backwater-region-kerala-55125235.jpg
LY symptoms on coconut:
https://bugwoodcloud.org/images/768x512/1504005.jpg,
http://idtools.org/id/palms/symptoms/images/fs_images/LY_Jamaica_2005.jpg and
http://www.cphdforum.org/wp-content/uploads/2015/06/LethalYellowingCoconutSymptom.jpg (fruit)

Links
Information on Kerala wilt:
https://www.researchgate.net/publication/338946394_Identification_and_confirmation_of_hotspot_areas_and_management_ofroot_wilt_disease_in_coconut,
https://www.thehindu.com/news/cities/Kochi/root-wilt-disease-proves-a-tough-nut-to-crack/article5086532.ece,
https://www.sciencedirect.com/science/article/pii/S0885576507001014,
http://www.apsnet.org/publications/plantdisease/2010/May/Pages/94_5_636.2.aspx,
https://srjcoconutconsultancy.com/root-wiltor-kerala-wilt-disease/,
https://www.researchgate.net/publication/236612608_Variations_in_hosting_beneficial_plant-associated_microorganisms_by_root_wilt-diseased_and_field-tolerant_coconut_palms_of_West_Coast_Tall_variety, via
http://archives.dailynews.lk/2010/12/28/fea20.asp, and
https://allaboutkerala-anishpanthy.blogspot.com/2008/10/major-diseases-affecting-coconut.html
Review and diagnosis of palm phytoplasmas:
http://scialert.net/fulltext/?doi=ppj.2010.112.121
KW phytoplasma taxonomy:
https://www.uniprot.org/taxonomy/212371
Other phytoplasma taxonomy via:
http://www.uniprot.org/taxonomy/33926
Phytoplasma resource centre:
https://plantpathology.ba.ars.usda.gov/phytoplasma.html
- Mod.DHA]