I'm a little unsure of which things in the EVElopedia are canon and which aren't, but in addition to the brief article on
Kresh trees there's also the clearly-canon article on
Heart of the Forest: "Heart-of-the-Forest is the spirit of the mighty kresh trees, of things that are green and growing".
Neither of these sources directly mentions the toxicity of the plant, but the section on
Tea - Caldari gives us:
Traditional Caldari tea is prepared from the leaves of the kresh trees that can be found in the Kaalakiota Peaks on Caldari Prime, the home of the precursors of the modern Caldari known as the Raata. It has a strong and very bitter taste, and is an integral part of traditional Caldari culture. Practices like the Tea Maker Ceremony, for example, are carried out by citizens and corporate heads alike, used as a form of divine judgement with potentially lethal consequences. Kresh tea is thus regarded as divine amongst the Caldari [....]
I read this as strongly suggestive of the toxicity of kresh, but not conclusive. The article on the
Tea Maker Ceremony itself refers to "poisoned tea" rather than "poisonous tea", carrying the possible implication that poison has been added to the tea rather than that it's an inherent part of the tea.
Personally, I've been enjoying seeing the development of the Caldari relationship with kresh. Kresh is the protecting mother-figure which can also poison you if you're not Caldari enough. I'd previously found the assertions about how only Caldari could drink it to be a bit on the implausible side, but the idea of gene-modding the early colonists to make them more compatible with the biosphere of their harsh new planet makes an appealing kind of sense to me. I can see how the rituals around kresh could develop out of that -- Herko describes it as part of the coming-of-age ceremony -- and how they would in turn select for people who were better-able to tolerate kresh.