Blackening of faba beans


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Hi all,

An extremely brief intro - entomologist from QLD in Aus, desperate for advice. I have a persistent issue of blackening/necrosis on faba beans that I am growing for use in insect cultures. I have little experience in horticulture and have struggled to find solutions online. Bean seeds are nasma variety, soaked for 24 hr and then planted into fairly sterile media. Temperature 24C, RH 50-60%, grown under LEDs at 4000k (colour temp). Blackening occurs within a few days of germination. Originally I thought it was overwatering, so I reduced watering to every 12 hrs (down from every 8 hrs), and it didn't seem to help much.

I know that I will not get optimum growth from these faba beans since I am limiting their nutrients by growing in almost sterile media (vermiculite and some peat). I have seen that they grow much better in media supplemented with potting mix. However, I have successfully grown the beans without supplementation and no blackening on a few occasions and not sure what I was doing differently. Please disregard the aphids in some photos, the blackening occurs without aphid infestation as well.

I'd like to break it down and make sure I am doing the basics correctly. I want to avoid adding too many things into the mix as some of our insects are incredibly sensitive to chemicals, even fertilisers. Any advice/questions will be appreciated. Thanks!
 

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IMO you are keeping your medium too wet. Both peat and vermiculite retain moisture. Blackened roots and stems are classic symptoms of root rot in plants which is caused by too much moisture. As far as nutrition is concerned the seeds themselves will provide enough nutrition for growth to the second set of true leaves although nutrition should be provided at the first set of true leaves. Have you tried an oscillating fan to remove surface moisture after sprouting? There appears to be a fungus on the soil also and this can be controlled by using horticultural cornmeal on the soil surface. This will allow Trichoderma fungi to flourish which in turn will destroy harmful fungi. The cornmeal will also help control the root rot fungus
 
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The method worked earlier but now a new pathogen has contaminated the growing environment.
How long do you need to maintain each insect culture before you can sterilize everything and start over?

Additional sterilization methods could include cooking or autoclaving the media and using a sterilizing solution on the seeds before germination. Of course, that could affect faba bean's symbiosis with N-fixing bacteria, assuming that symbiosis is functioning in this case.

The two usual prongs of attack would be to kill the pathogen and to improve the health of the plant, but it sounds like those options are limited in this case. The third prong is keeping the pathogen out of the environment, which may be the most difficult. Keeping the colony in a enclosed growth chamber would make this easier, but that might not be feasible.

Are you raising aphids to serve as a host or food source for another study organism?
 
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IMO you are keeping your medium too wet. Both peat and vermiculite retain moisture. Blackened roots and stems are classic symptoms of root rot in plants which is caused by too much moisture. As far as nutrition is concerned the seeds themselves will provide enough nutrition for growth to the second set of true leaves although nutrition should be provided at the first set of true leaves. Have you tried an oscillating fan to remove surface moisture after sprouting? There appears to be a fungus on the soil also and this can be controlled by using horticultural cornmeal on the soil surface. This will allow Trichoderma fungi to flourish which in turn will destroy harmful fungi. The cornmeal will also help control the root rot fungus
Hi Chuck,

Gotta say I am super excited that two people have already replied to my thread!
I also thought it was too much moisture the first time I saw this blackening. Unfortunately the opinion of a few others was that the plants were not getting enough water, so we went down the route of increasing water and things got worse before they got better. Literally yesterday I installed a dehumidifier in the room and set it to 50% as the average humidity was hovered around 75% and even I knew that it was too high. So this dehumidifier is providing some airflow when it is running, and the airconditioning system also has a fan when it's on.

I'm hesitant to add even a cornmeal substance since the beans are only used for about 2-3 weeks before they are harvested. Although, this bean culture is split into two and in one of them we are learning how to compost. So perhaps for this one we could try the cornmeal.

I have a question about initial watering - after soaking and planting the seeds I find they do better if I leave them without water for a few days. But sometimes if I leave them for even an extra day longer without water it greatly stunts the germination. Is there an optimum period you'd recommend for this? I.e. should I plant the seeds, wet the medium, and wait a few days before watering again? Or maybe plant them without wetting the medium and water them sooner? Thanks for your help, just realised I'm now 10 minutes late for work in typing this so I hope it doesn't seem rushed!
 
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The method worked earlier but now a new pathogen has contaminated the growing environment.
How long do you need to maintain each insect culture before you can sterilize everything and start over?

Additional sterilization methods could include cooking or autoclaving the media and using a sterilizing solution on the seeds before germination. Of course, that could affect faba bean's symbiosis with N-fixing bacteria, assuming that symbiosis is functioning in this case.

The two usual prongs of attack would be to kill the pathogen and to improve the health of the plant, but it sounds like those options are limited in this case. The third prong is keeping the pathogen out of the environment, which may be the most difficult. Keeping the colony in a enclosed growth chamber would make this easier, but that might not be feasible.

Are you raising aphids to serve as a host or food source for another study organism?
Hi Marck,

I am keeping the beans active for 2-3 weeks, 4 at most, and then they are composted or thrown away. The aphids are used as a host, so their health is also important. So I am able to start fresh plantings quite regularly and clean things thoroughly. I also replied to Chuck's post and maybe some of that info will be useful to explain my situation? Thanks so much for your advice so far!
 
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Hi Chuck,

Gotta say I am super excited that two people have already replied to my thread!
I also thought it was too much moisture the first time I saw this blackening. Unfortunately the opinion of a few others was that the plants were not getting enough water, so we went down the route of increasing water and things got worse before they got better. Literally yesterday I installed a dehumidifier in the room and set it to 50% as the average humidity was hovered around 75% and even I knew that it was too high. So this dehumidifier is providing some airflow when it is running, and the airconditioning system also has a fan when it's on.

I'm hesitant to add even a cornmeal substance since the beans are only used for about 2-3 weeks before they are harvested. Although, this bean culture is split into two and in one of them we are learning how to compost. So perhaps for this one we could try the cornmeal.

I have a question about initial watering - after soaking and planting the seeds I find they do better if I leave them without water for a few days. But sometimes if I leave them for even an extra day longer without water it greatly stunts the germination. Is there an optimum period you'd recommend for this? I.e. should I plant the seeds, wet the medium, and wait a few days before watering again? Or maybe plant them without wetting the medium and water them sooner? Thanks for your help, just realised I'm now 10 minutes late for work in typing this so I hope it doesn't seem rushed!
Cornmeal is best when used prior to and at seed planting. After seeds have already sprouted it is a little late for beneficial fungi to grow and be of great help. Cornmeal has no effect on the plants themselves. All it does is give the Tricoderma fungi its favorite growing medium. Peat and vermiculite both retain water. Both help with oxygenation of the growing medium BUT, too much water too often will cause root rot fungus. Soaking beans for 24 hours is too long IMO. 12 hours max or overnight is what I do. Optimum medium and ambient temperature should be 27C. Before planting the soaked seeds completely saturate the growing medium and let it drain. Then place your seeds on the soils surface and lightly cover the seeds with dry medium. Cover the container tightly with Saran Wrap. This will act as a miniature greenhouse and it will stop evaporation. As soon as seeds germinate remove the Saran. Do not water at this time. As soon as the cotyledon leaves unfurl mist with a spray bottle until the surface of the soil is wet. Do this once per day as long as the dehumidifier is running. Before misting feel the soil. If it is only slightly damp the plants do not need watering. Remember that the peat and the vermiculite have retained a lot of water. After misting for about a week it will probably be time to actually water the seedlings. Water from the bottom up. Just place the growing container into water and let it soak up the water. When water stands on the surface, remove and let it drain. Do not water again until the medium is DRY on the surface to a depth of about 1/2 inch. The roots will have grown deeper into moist soil by this time. Then water from the bottom up again.
 
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