You don't apply an ability score to monster attacks unless stated.
The attacks on a monster's page contain everything you need when rolling for that monster.
In this case, the Rotting Touch attack doesn't use an ability score to attack. It has a flat +2 to hit, followed by +0 to damage, and nothing more or less. You would roll a d20 and add 2 to the result for the attack roll. On a hit, you would roll a 1d8 and add nothing to it, and that would be the damage.
Know why it's not useful.
There are a couple of ways to approach the problem of presenting the PCs with information that is interesting, and not useful, and that is up to them to make useful.
They all start with coming up with something interesting, and it doesn't sound like that's a problem for you.
If the interesting thing is useful, come up with an obstacle to its usefulness that the PCs will have to navigate.
If the interesting thing is not useful, come up with a way the PCs can make it useful right now, or a scenario where it might be useful to the PCs, and keep that in your back pocket for when you need to prompt a PC to do something.
Barriers to Usefulness
So let's start with some useful information:
Ingredients that would otherwise be rare are abundant and cheap.
How can you make this not immediately useful?
You could introduce something else people would have to find out:
As long as you can identify yourself to the stallholders as a chef and not just a traveler looking for a cheap meal. Now, how could you do that?
You could introduce a cost to pay:
As long as you're in good standing with the Guildes des Cuisines, which famously charge a 500-coin lifetime membership fee.
Or an obstacle to overcome:
As long as the market chief, Gastrome de l'Etoiles, personally approves of your cooking.
On a 6- you might consider severely ramping up one of these costs if you think it would go hard enough:
As long as you're in good standing with the Guildes des Cuisines, which are famously headquartered right where this giant smoking ruin is now. ...oh dear.
Pathways to Usefulness
Or suppose you come up with something interesting that doesn't look immediately useful. It's still got to present some hook - it's your play in the back-and-forth narration, so you should be setting the players up somehow.
Well, unless you're running a white-knuckle first session, the PCs don't have exactly one problem. And you knew the problem they were Spouting Lore about in the first place, right? Otherwise you'd have no idea what to say to be useful.
So look at all the other problems out there - all the fronts, and all the things PCs have cared about. If you can make it fit as a stepping stone on one of those, all well and good.
If not? Well, if you feel like it won't dilute things too much to open up another front or another ongoing project, go for it.
Best Answer
If the time is running backward, it is weird that the players start in a "future" state. They should be as they are when they enter the pocket. But well, it's a backward time pocket so don't have to be consistent on that !
Playing a full scene with the time running backward will be pretty difficult. Because it mean that everything the players will do, they already did it. So if a player start with a mushroom in his hand, at one time the mushroom will be in his hand. Even if the player does nothing it should happen. You have a high chance that the players just do nothing and wait.
And for the players point of view, doing an action in a reverse time world will be near to impossible. What are they going to do anyway ?
If their goal is to get that mushroom, there's no way a player will do that. Because nothing is saying to the players that if they play the scene backward they'll leave that time pocket with the mushroom.
Here's some suggestions:
The players enter the time pocket as they are. The time is running "normal" for the players but the time is running backward for everything else in the time pocket (the tree, the mushrooms, ...). Because of this, if they want the mushroom, they'll have to get it and leave before the mushroom just stop existing anymore.
The players has to guess what happens during the scene and they really have to play the scene backward. If they fail, the time pocket is broken and collapse.
The players can't move and they have to look at a scene running backward. Then the scene start again and the players can move.
I tried the first mechanic in a game. The players goal was to recover an old artefact (destroyed many years ago). They found a specific spell which can copy the artefact and then a powerful mage helped them. The mage sent the players in the past at the creation of the artefact during a battle between 2 armies.
The players appears near the artefact location but the world was still "running backward". So they had to reach the artefact, dealing with a running backward world, to copy the artefact because it was never created.
There was some specific mechanics for this scene:
The players understood fast that the world was still running backward but no them. So they should act quickly before it was too late (or too soon).
Running a backward battlefield is weird for the GM, but it's fun.