mold on seeds in petri dishes

There have been some comments about getting mold in zip lock bags or petri dishes in spite of using hydrogen peroxide. Hydrogen peroxide appears to be effective against the molds that cause damping off. It will not kill all types of molds.

Sometimes I will get a rapid formation of white mold that completely covers the inside of the petri dish. After a time the mold disappears as fast as it appeared. Often I get germination soon after the mold collapses. Others have made similar comments that they find that the molds help germination. When I use the blender to process large numbers of seeds, I often have pulp in the petri dish with the seeds. I have sent seeds in that condition to others and have received back comments that the seeds were moldy but germinated very well in the mold (and were not damaged by the mold).

I would be reluctant to use bleach to kill the molds that appear in the petri dishes or zip lock bags for 2 reasons:

  1. if the seed coats have already formed cracks, you may kill the seeds

and 2) plastics adsorb chemicals, the residual bleach could cause problems later for the emerging seedling.

Title: Disinfection of rice seeds prior to sprouting.

Authors: Piernas, V.; Guiraud, J. P.

Authors affiliation: Lab. Genie Biologique Sciences Aliments, Case Courrier 23, Univ. Montpellier, 2 Place Eugene Bataillon, 34095 Montepellier Cedex 5, France.

Published in: Journal of Food Science, volumn 62, pages 611-615, (1997).

Abstract: “Different methods of rice seeds disinfection were investigated in order to limit the development of micro-organisms during sprouting. At room temperature, sodium hypochlorite at 1000 ppm or hydrogen peroxide resulted in a 2 to 3 log decrease in aerobic plate counts. Ethanol was found to inhibit germination, thus prohibiting its use. The limited effectiveness of decontaminating solutions could be because they did not access the bacteria, which was supported by the results of washing with benzalkonium chloride. Greater reduction (up to 5 log) was obtained by soaking seeds for 5 min in a sodium hypochlorite solution at 60 degree C.”



Author: JAMES R L

Published in: U S Forest Service Northern Region Cooperative Forestry and Pest Management Report. (85-23) 85-23 1986. 1-9.

Abstract: “Pathogenic Fusarium on spruce seed from the Towner nursery, North Dakota [USA].Five seedlots of Colorado blue spruce [Picea pungens] and three seedlots of Black Hills spruce [P glanca var. albertiana] were sampled for Fusarium contamination. All seedlots contained some seed and/or debris with Fusarium. Levels of contamination were greatly reduced by treating seed with running water rinses for 48 hours or with chemical sterilants such as sodium hypochlorite (bleach), hydrogen peroxide, captan, and benomyl. All three species of Fusarium commonly isolated from seed, F. oxysporum, F. solani, and F. tricinctum, were pathogenic to blue spruce germlings after test tube inoculations. Reducing the amount of seed contamination will help reduce future losses from pre- and post-emergence damping-off.”


Title: Germination of seeds and damping-off and growth of seedlings or ornamental plants as affected by soil treatments.

Author: Doran, Wm. L.

Published in: Mass. Agr. Expt. Sta., Bull. (1938), 351 44 pp.

Abstract: “Damping-off disease, caused principally by species of Rhizoctonia, is not altogether controllable by regulating and adjusting the soil temp., moisture and pH, for conditions preferred by the fungi are too nearly the same as those preferred by the plants. Damping-off was not controlled by CaSO4, Ca(OAc)2 or CaCl2 or by materially raising soil pH values with Ca(OH)2. The disease was well controlled by CaCN2 and also by NH4OH. NH4OAc was less effective. NH4CNS was effective but remained in the soil too long and exerted a toxic action on the plants. HCOOH and salicylic acid gave good control and were not usually toxic when well worked into the soil. Tannic acid was not fungicidal. AcOH dusts controlled damping-off and were not injurious to seedlings when applied to the soil the day before planting. They were very toxic to soft-wood cuttings, however. Quantities of AcOH, CH2O and other volatile chemicals which were safe if worked into the soil immediately before seeding were injurious if applied to the soil immediately after seeding, for they are then more concd. near the seeds. Crucifers were more often injured by AcOH and CH2O than most other plants. AcH was less toxic to both plants and fungi than was CH2O. Sweet pea seed germination was benefited by the addn. to the soil of either AcH or EtOH. Heavy application of KMnO4 or Ca(ClO2)2 did not control damping-off. Cu-lime dusts were useful for seed treatment but were toxic for many species of plants. Al2(SO4)3 injured many seedlings but was perfectly safe with Dianthus. Both Dianthus and Rhododendron were notably tolerant to H2SO4. S did not act as a good fungicide.”

The link below first uses a H2O2 treatment then a biological control:


Good info, Henry. Thank you for the abstracts! As you pointed out, some fungi are not harmful (and may be beneficial) to the seed germination process. Unfortunately, some are not, and it’s difficult to selectively kill the bad and save the good. As I am more interested in killing off the spores and mycelium before the seed germinates, I feel I can be fairly agressive in using anti-fungicides like bleach, esp. since the rose seed coats are so impermiable (except to fungi, of course). I guess we’ll see. I’ve now got a lot of seeds to play around with, thanks to donations from other Rose people. Let’s see… bleach, H2O2, Lysol, Tinactin, Chicken Soup…

If you decide to use the combined first H2O2 then biological control, see my write-up for a source of a biological control at:


Thanks. I wonder how it would do on paper towels? I did find their website and I will check it out.

If you sow the seeds before they start to germinate, then it isn’t a problem to allow the seeds to mold. In fact, I have evidence that suggests that almost any mold is useful in breaking down the hard shell of the achene and encouraging the imbibition of water.

I collect seed hips in September/October and place them in the fridge until late December/early January, and then I start removing seeds from the hips. This gives only 8 weeks to accumulate molds, but not long anough to allow significant germination. I absolutely believe that germinating the seeds on paper towels is a poor idea, and often results in the death of the seedling. (From personal experience) They are meant to germinate in a soil type media.


I think it all comes down to a space issue. For those with lots of space, it makes sense to plant the seeds before they germinate. But for those with limited space, germinating seeds on paper towels and planting only those that germinate, takes less space than planting in big trays.

Paul, do I read your post correctly? You stratify the seeds while still in the hips?

Correct, the seeds are stratified while still in the hips, at least for the first 2 months of it.

Henry, great list of abstracts here. I was particularly intrigued by the H2O2 followed with a biological control. I have become quite a “fan” of the use of H2O2 this year with my own experiences. Unfortunately I was not able to access the USDA publication on this tonight. I will try again tomorrow…however, I was wondering if they discussed comparitive effectiveness of various durations of H2O2 exposure. I have begun to suspect that total emersion for up to 24 hours is more effective than perhaps shorter durations of emersion for up to 6 - 10 hours.