Ph.D

Disease free planting stock significantly ensures better field survival, thereby
reducing the burden of economic loss and meeting the targets of climate change
mitigation. Teak (Tectona grandis L.f.), Indian beech (Pongamia pinnata (L.) Pierree),
custard apple (Annona squamosa L.), guava (Psidium guajava L.), and Indian
laburnum (Cassia fistula L.), which are extensively used for afforestation programs in
Kerala, are severely affected by fungal pathogens at the nursery stage. Hence, an
investigation was undertaken to identify the major fungal diseases associated with the
above mentioned seedlings in Kerala and to assess the efficacy of selected fungicides
and bio-control agents through in vitro and in vivo studies.
A purposive sampling survey was conducted in 40 forest nurseries across the
state during summer (Feb-May), monsoon (June-Sept), and post-monsoon (Oct-Jan)
seasons in 2021 to monitor the occurrence of fungal diseases and collect diseased
samples. Out of the seven major fungal diseases, three from teak, and one each from
the other four tree crops were observed respectively. Major fungal diseases noticed
from teak seedlings include teak leaf rust (TLR), teak die back (TDB), and teak leaf
spot (TLS). Pongamia foliar disease (PFD) from Indian beech, annona die back (ADB)
from custard apple, guava leaf spot (GLS) from guava, and cassia die back (CDB)
from Indian laburnum are the other selected major diseases. Teak leaf rust (TLR) was
the most common and severe foliar disease observed in teak from various nurseries
across the state and irrespective of the five agro-ecological zones (AEZs), the
maximum Percent Disease Index (PDI) and Percent Disease Severity (PDS) (60.0%
and 52.8%, respectively) recorded from Chettikkulam (AEZ IV- High Hills) in the
monsoon season. Teak die back (TDB) disease (PDI 62%) was observed only from
Edayazham (AEZ I- Coastal Plain) during the pre-monsoon period. Teak leaf spot
(TLS) was noticed only from Kudayathoor nursery (AEZ III- Foothills) during the
pre-monsoon season. In Indian beech, PFD was observed from different nurseries
irrespective of AEZs during the study period, and Chettikkulam (AEZ IV- High Hills)
recorded the maximum PDI (80.0%) and PDS (63.2%) in the monsoon. The major
disease of custard apple seedlings, i.e., ADB, was observed from different nurseries,
and the maximum PDI (55.0%) was recorded at Kanakappalam (AEZ III- Foothills).
Guava leaf spot (GLS) was noticed as the common disease in guava seedlings during
the survey, and the maximum PDI (60.0%) and PDS (55.0%) were recorded at
Varanadu nursery (AEZ I- Coastal Plain). In Indian laburnum, CDB was found to be a
common disease throughout the state, and at the location Kattamudy (AEZ IV- High
Hills), it recorded the maximum PDI (50.0%). Based on the available data during the
study period, TLR showed a positive correlation with temperature and a negative
correlation with rainfall. However, PFD showed a negative correlation with
temperature and a positive correlation with rainfall and ADB showed a positive
correlation with rainfall as well as relative humidity. In the case of GLS, no common
trend was detected, and CDB showed a negative correlation with relative humidity.
The typical orange colour urediniospores on the backside of the affected teak
leaf characterizes TLR symptoms. Meanwhile, TLS is noticed as scattered black spots
with a greyish center. Whereas TDB originates as leaf blight and subsequently spreads
throughout the seedlings. Symptoms of PFD are observed as irregular chlorotic spots
on the adaxial side and black dot-like sporodochia on the abaxial side. Leaf yellowing,
followed by inward curling and the further spread of a necrotic area throughout the
seedling, is the typical symptom of ADB. Meanwhile, GLS is noticed as grey to
cream-colored blighted region surrounded by black, brown, or violet round margin. In
the case of CDB, the initial symptom of leaf blight later spreads throughout the
seedling. The pathogenicity of the major foliar diseases (TLR, TLS, PFD, GLS) was
proven using Mycelial Bit Inoculation Method (MBIM), Micro Droplet Inoculation
Technique (MDIT), and inoculation with the infected leaf method. Spore suspension
method and mass multiplied pathogen in the substrate method were adopted for major
root diseases (TDB, ADB, CDB). The obligate parasite (TLR) was identified as
Olivea tectonae based on the morphological characters of urediniospores and all other
major pathogens were identified upto the species level based on cultural as well as
morphological characters, coupled with characters of amplified gene regions (ITS
region and LSU region). The TDB pathogen was thus identified as Colletotrichum
fruticola, TLS pathogen as Corynespora cassiicola, PFD pathogen as Cladosporium
tenuissimum, ADB pathogen as Colletotrichum siamense, GLS pathogen as
Colletotrichum siamense, and CDB pathogen as Alternaria alternata.
In vitro evaluation studies revealed that carbendazim 12% + mancozeb 63%,
mancozeb, and propineb stood out against major foliar diseases. However, in vivo
experiment results showed that carbendazim 12% + mancozeb 63%, mancozeb,
hexaconazole, and Trichoderma asperellum were observed to be efficient against
major foliar diseases. Similarly, carbendazim 12% + mancozeb 63%, mancozeb,
Bordeaux mixture, and hexaconazole showed promising results against major root
diseases during in vitro as well as in vivo experiments.