0. Time your search

HpA prefers cool temperatures and high humidity to sporulate – so the ideal days to spot ‘white powder’ is at day temperatures around 15˚C, several days after night temperatures did not drop below 5˚C, and 3-4 days after there have been a few days of rain. Most plants will not have flowered at this point, so they need to be identified by characteristic features of their rosette.

1. Find Arabidopsis thaliana

While Arabidopsis is common and wide-spread, it does not grow ‘everywhere’. To hunt for plants, you may try to find again sites previously described, such as from the 1001 Genomes germplasm or the Global Biodiversity Information Facility.

2. Other plants not to confuse Arabidopsis with

3. Spotting HpA

HpA sporangiophores primarily grow out of stomata on rosette leaves, so it is the underside of leaves that need close inspection. Infected leaves are often unhappy, and have tell-tale yellow margins. In case plants are already flowering, stems and siliques are also common locations of ‘white powder’.

To avoid the unnatural spread of disease, tools such as tweezers should be sterilized between touching plants – e.g. with alcohol wipes.

4. Other pathogens that should not be confused with HpA

Another common oomycete pathogen of A. thaliana is Albugo sp. While HpA forms a rather uniform layer of ‘powder’ on the underside of a leaf, round pustules are observed for Albugo. Mixed infections are not uncommon.

5. Harvesting infected plants

Before harvesting plants, please ensure that the A. thaliana population at the site you are visiting is large enough to survive even when some individuals are removed. If yes: harvesting more than 1-2 plants per site is not needed.

Also make sure that you are not in a protected area, where sampling plants requires special permits.

Use a spoon or tweezers to remove the entire plant from the soil, including roots and some attached soil. Place the plant in a sufficiently large container, e.g., a 50 ml Falcon tube.

6. Metadata

Together with plants, we need:

• Date of collection
• GPS data of site of collection – at beast marked as a spot in Google Maps – see example above.
• One or more picture(s) of the infected plant ('powder' visible)
• Also good, but not essential: picture(s) of the collection site

7. Sending plants

Keep plants in the cold (in the fridge, not in the freezer) until shipping.
Send us an email, including the picture(s) you took of the infected plant(s): pathodopsis@tuebingen.mpg.de.
If you work at a University, we can easily arrange a Fedex pickup.
If you don’t, we will find out how else to organize transport.

Thank you for your help!!!