Choice  of  elastomer for  the  sidewall compound is  of  great  importance in  determining the optimum balance  of properties  required to give  adequate resistance to all forms  of cracking, especially in radial  ply tyres. Natural rubber alone may not fulfil all the requirments and hence the use of blends of natural rubber with  other  elastomers such  as polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. has become  common. The present study is an  attempt to evolve  a suitable mixing  procedure giving  improved processability of the mix.  Seven  different methods of mixing  have  been  tried,  keeping  the ratio of natural rubber  and  butadiene rubber constant. Parameters such  as  viscosity,  die  swell  and  stress  relaxation were  used   to  assess processability. BR masterbatch (Mix E) method was found  to  be the  best.

Efficacy of nine fungicides as protectants and  eradicants against  Oidium heveae was tested  in the laboratory. Sulphur  as  dust   and  wettable and  flowable formulations  were   better   than   the systemics tridemorph, triadimefon, triadimenol, prochloraz and  benomyl in inhibiting infection and  totally  suppressing sporulation when  applied as prophylactic. As antisporulant, sulphur was as effective  as triadimenol and  tridemorph even  when  applied a week  after  inoculation. Only f!owable sulphur was as persistent as the systematic fungicides. Field trials confirmed the effectiveness of  tridemorph. Good  control of pink  disease was  achieved   with  two  sprays at  two  weeks  interval of' either aqueous mixtures of 2% a.i. chlorothalonil (Daconil   2787 50 WP) with  a sticker  (Tenac)  and  a surfactant (Megapol 440), or 3% a.i. of the commercially prepared Bravo 500, or Daconil flowable 500. Brushing with  Daconil  2787 50 WP  or  Thiram  80 WP  at 3%  a.i formulated in  bitumen emulsion was equally  effective as the commercially prepared Calixin readymixed. Better disease control was obtained when light  or  moderate infection  was  treated.

Evapotranspiration (ET) of immature rubber  was quantified  and crop coefficient (K) worked out using non-weighing  type of lysimeters.  ET, showed  wide variation depending on weather conditions,  the mean value during  December to April being 4.97 mm d^-1 and  during May to November,  2.99 mm d^-1 The mean K_c values were 0.93, 0.82 and  0.77 for  pan  evaporation, Penman  and  Hargreaves  methods  respectively.

Natural   rubber is easily attacked by ozone, affecting strength, surface finish and durability of rubber products, especially of those used outdoors. Blending of EPDM with superior processing natural rubber was attempted to develop   natural rubber compounds with  improved ozone resistance  and  extrusion  properties.  A blend of EPDM and SP-20 in the ratio 30:70 showed optimum properties.  Reinforcement of the blend using  fEF black and  the effect of china clay and surface modifying  agents DEG, TEA and Si69 in the filled compounds were studied. The blending resulted in better ozone resistance, extrudate finish, reduced die swell and moderate mechanical properties.

The  biochemical changes associated with  tissue  browning in  in  vitro  plant  culture and   the influence of medium conditions and explant  origin on  callus browning were studied. Some antibrowning agents were also tested.  The  activities  of  peroxidase (POD),  soluble and   bound polyphenol oxidase (PPO_s and PPO_L), NADH-quinone reductase (NQR), superoxide dismutase (SOD),  catalase   (CAT)  and   glutathion  reductase  (GR)  were  studied. As the  culture  aged, browning was accompanied by a marked increase in POD, PPO_S, PPO_L and SOD activity.  NQR was higher  in the seeds  and  showed little variation after 10 days  of culture, whereas high  CAT activity  in the explants decreased considerably  to the point that it was difficult  to measure after 40-50 days' culture. The concentration of hormones in the callogenic induction and  the embryo initiation  media   were   found  to  be  a  crucial   factor   in  browning  and   also  controlled the embryogenic potential. At lower hormone concentrations, browning was always accompanied by a marked increase in POD, PPO_S, PPO_L and  SOD activity  and  although  pale-coloured  calli obtained at these concentrations were 100% embryogenic, there were more brown  calli than pale calli. Calli obtained in  media  with  low  hormonal content  and  the corresponding pale  highly embryogenic calli were characterised by very high SOD activity. Calli from  different seed  parts (funicle, median  or distal) reacted differently at different  hormone concentrations in the medium. A study of 7 standard clones with different browning potential revealed that the brownest clones generally showed high  PP0_S  PPO_L  POD and GR activity. CAT activity  was found to be higher in the least  brown  calli than in   pale calli or slightly  necrotic, non-embryogenic calli.  A certain number of  culture medium additives effectively  reduced  browning. Phenol     traps,  such   as polyvinylpyrolidone (Polyclar  AT, PVP), an absorbant resin  (Amberlite XAD) and  antitoxidant agents, such  as  ascorbid acid,  were  able  to  reduce   tissue  browning and  favour growth of embryogenic calli  to various degrees.

Impact  of the  input subsidy scheme,  introduced by the  Rubber  Board  in 1986-87 on  cultural practices of smallholdings was  analysed, based  on  a multi-stage selective  random sampling survey covering 5 regions  and  207 member  growers attached to 25 Rubber  Producers Societies. Wide   inter-regional  variations  on  account   of  region-specific factors  were  evident.  Lack  of awareness about improved techniques and  resources were the main  constraints during the pre­ scheme period. All the  thirteen  subsidised inputs were  availed  of only  in one  region.  Among the inputs chemical fertilizers was  the most  popular. Although the net  impact of the  scheme on  yieid  could  not be assessed , the study  has identified the pattern of changes in the cultural practices. The net  cost saving  on  account  of a shift  in favour  of straight fertilizers during the post-scheme  period   was   Rs.  326  per   hectare.   The  additional mandays generated during 1988-89  was  34.19  and   estimated subsidy  was  Rs.  681.87  per   hectare.   Constraints in  the application of subsidised inputs were identified and need for extension  services in selected cases outlined.