Black Growth on Eucalyptus Seedlings (E. gunnii)

Question

I have a number of eucalyptus seedling that are still quite small (from one to five centimeters). After bringing the ones I had outside during the summer back indoors most of them got a black fungus looking growth on their leaves and started dying. The growth starts at the growing tip and works its way down. In the end it kills all the leaves and just the tiny stem is left, which is still too small to produce new leaves.

At the around that time one of the pots of seedlings that I had kept indoors got the same black growth. It happened around the same time I brought the other seedlings in but the pots where never in the same room.

I think that the black growth is some kind of fungus and that its onset has something to do with the lower light levels present indoors. What confuses me is that it only seems to be effecting E. gunnii and some other mixed eucalyptus species that I sowed with them. The E. cinerea seedlings I have (I have a lot of Eucalyptus seedlings) are doing fine and I have not seen any black growth on them.

Has anybody experienced something similar? I am looking to grow more eucalyptus species from seed in the future but want to know if there's a way to prevent this before I do it. At this point it has killed all but a few of the biggest E. gunnii seedlings I have.

So far the only thing I can think to do different is plant them earlier in the season so that they can stay outside longer and get more woody by the time I have to bring them indoors. I have a year+ old eucalyptus tree right next to the ones effected by the black growth but it has not shown any signs of infection, so I am guessing it's only a problem for the seedlings.

Answer 1

If we examine the list of diseases that Eucalypts are commonly affected by we find that Phytophthora is quite an important member. It is associated with high heat and excess moisture, so if you have been generous with the watering it might be relevant. There are lots of images online of the effect of this fungus, and Eucalypts are only one example. Your description does not fit exactly; you state that foliage turns black, but brown is a colour often cited, and since Phytophthora usually affects roots first then unless your observation about the top down spread is a secondary infection after the primary in the root then maybe you are spared this rather nasty pathogen. Evidently in many genera there is varying susceptibility according to species and variety which may explain your other observations.

Pics are always helpful; if you cannot upload one then find a similar image from an image search and edit your post to contain the link. It would help a lot. In this case if you can take the plant out of the pot and show detail of root condition that would be valuable additional material.