33.1 I am here too: New approaches to the control of lichen growth


An article published by Tretiach M., Bertuzzi S, Candotto Carniel F.; "Heat Shock Treatments: A New Safe Approach against Lichen Growth on Outdoor Stone Surfaces” Environmental Science and Technology, 2012; reports about a new simple approach to the control of lichen growth, based on these factors:
  • the application of a biocide
  • keeping t =50°C (122 °F) for 4hrs
  • Keeping the lichen wet all through the treatment
Note: If we dry up the lichen at that temperature, the application doesn't work. There are evidences these organisms are thermal tolerant when dry, so that they turn into dormant bodies, and they revive as soon as environmental standard conditions are reestablished. The test was carried out at t=20°C (68°F) and at intermediate t=40°C (104°F) in order to evaluate the effect of temperature on the results of the treatments. The combination biocide + warming temperature + moisture enables to reduce the concentration of biocide, (ten times lower than those ones in current use), with significant benefits. Biotin T, Biotin R and New Des 50 have been tested.

1_The first screening, conducted only on a specific specimen, Verrucaria Nigriscens, shows different outcomes depending on the chosen temperature (t˳= 20°C/68°F; ta=40°C/104°F and tb=55°C/131°F)after conditioning dried or hydrated vegetative tissues for 24 hrs.

Without biocides 
At t˳=20°C (chosen as standard normal conditions), thallus (the body of lichen) grows rapidly. At ta=40°C there is a partial inhibition of micro-organisms vegetative functions. In spite of that, once temperature is lowered to 20°C, lichen starts to grow at the normal rate again. This phenomenon (inhibition-resumption) has been observed even on dried samples, conditioned at tb=55°C. Such a heat shock treatment is effective ( long-term inhibition) only on the moist samples.

With biocides 
The best results had been obtained when applying New Des 50 at 5%. As a matter of fact that product works well even at t˳=20°C so no more tests at severe heat conditions were required.

As a second step of the research, six species of lichen were treated with the two grades of Biotin, at the same temperatures of step 1, but only for 6 hrs. Under such a short thermal treatment, the outcomes were as follow: poor at t˳=20°C (only two species were affected by Biotin T); a little bit better at ta=40°C(Biotin T reacted well on five species); totally satisfactory at tb=55°C for both Biotin T and Biotin R.

Finally an outdoor test was carried out at the temperature in between 50°C and 53°C for 4 hrs. Some parts of the lichen sample were covered with both transparent and black plastic films; while some other parts were left disclosed to dry in the sun.
  • Disclosed areas of lichen are not significantly affected by the application of Biotin R (at different concentrations) or by other two commercial biocides at low concentrations. After a short while thallus revives again. To get a good result it is necessary to use New Des 50 at 5% or Biotin T at 2% (in this case achievements are a little bit lower)
  • After 18 days, any vegetative activities of the areas covered with the plastic film is completely pulled down, despite the grade of the biocide and its concentration. Black colored film seems to work better than the transparent one (maybe due to the levels of humidity and heat)

The outline: always focused on hydrated lichen. Note the dry lichen can stand the thermal shock process even at 55°C and revive just after 8 days.

of the                       no biocides                        Biotin R 4%                        Biotin T 2%                              New Des 50 5% 
Thermal shock in use 

24 hrs          t˳=20°C                     std. growth rate               
 significant decrease;              significant decrease;                stop with slight resumption                                                                                                    restarting slight resumption    back to the std rate                  after 16 days
                                                                                              after 8 days                            after 8 days
=40°C                  significant decrease             stop restarting                        stop restarting                               stop restarting 
                                                  slight resumption                  slight resumption                   slight resumption                           slight resumption
                                                  after 8 days                          after 16 days                          after 16 days                                 after 16 days

=55°C               stop and
                                               slight resumption                            -                                                  -                                                            -
                                               after 8 days 

6hrs             t˳=20°C                std. growth rate                      decrease                                 decrease                                                         -                                                                                                                     depending on the specimen    depending on the specimen                                       
                                                                                                restarting after 16 days           further slow down after 16 days 
=40°C              decrease depending                 decrease depending                      stop (except for one specimen)                -
                                              depending on the specimen     depending on the specimen           further slow downafter 16 days
                                              restarting after 16 days            (except for one specimen)
                                                                                               further slow downafter 16 days

                     t=55°C           stop for all the specimen              stop for all the specimen               stop for all the specimen                           -
                                            even after 16 days                      even after 16 days                         even after 16 days

                                                                                    BIOTIN R                                               BIOTIN T                                 NEW DES 50
                                                                                                   4% 0,4%                                         2%               0,2%                        5%         0,5%                           
4 hrs in between 50-53°C         slight decrease                      temporary decrease                        significant      decrease            significant   decrease
without plastic film                                                                                                                         decrease       and complete     decrease    complete
                                                                                                                                                                                recover                                 recover                                          

4 hrs in between 50-53°C         stop for all samples
samples covered with              slight restart                                      Excellent results with all the biocides, even at low concentration 
black or transparent                 after 8 days
plastic film                                 


All those remarkable findings enable restorers to optimize the effectiveness of the biocides mentioned before by the mean of simple actions
  1. Ensure to moisten the thallus before using solutions of biocides
  2. Keep the lichen wet by covering that with a protecting plastic film
  3. It is necessary to generate a thermal shock by raising the temperature up to 55°C and keep it for 4hrs at least

Concerning the last point, it is clear that 
summer is the best season for the treatments with biocides, because such high temperature is easy to get under the black plastic cover. If the operations are carried out in winter we recommend to use a heater to keep the air warm under the film.
These procedures allow a significant reduction of biocide, cutting the operations costs and risks for users.