useful

I actually have a Grave Cleric of Talona (Goddess of disease and poisons) and this is like his signature spell, it is incredubly useful for the Tomb of Anihilation setting but also as a means of collecting make-shift poison reagents. 

The lack of venomous creatures in the list is unfortunate. Im sured that is what they wanted to say in the end

1st this works well with a hombrew class im playing where my charachters weaknesses is death by combat and death by poison so this is handy 2 is the yew leaf consumed? or do i only ever need one

The yew leaf isn't consumed. It would say if the yew leaf was consumed.

If you would prefer not to need a yew leaf, you could use a spellcasting focus such as a wand or holy symbol, which makes it so that you don't need to provide material components without a cost requirement.

Venom is a type of poison secreted by animals, usually via bite or sting. So the wording of poisonous creatures in my mind covers both an animal like a frog that is toxic to consume and a snake with a toxic bite.

Quote from Soviet_Static >>

The lack of venomous creatures in the list is unfortunate. Im sured that is what they wanted to say in the end

 It would be fair to assume that poisonous creatures are any creature that has a natural method of dealing poison damage. Also given that the DMG states "A character can instead attempt to harvest poison from a poisonous creature, such as a snake, wyvern, or carrion crawler." and that the DMG defines poison types as contact, ingested, inhaled and injury, it should be safe to say that venomous creatures are classified as poisonous creatures, regardless of real world definitions of poison and venom.

Great to use at parties to watch people's drinks to see if any get drugged.

Fun Fact: This spell has a materiel component that doesn't exist, as yew have needles instead of leaves. BTW, I have one in my backyard, so I know.

Are you able to use this if you are a Half Elf Wizard with the Mark of Detection or is it only for those specific classes still?

Pine & yew needles are a type of leaf, yes. But they are usually called needles, not leaf, due to the shape and structure.

Does the disease detection only detect creatures or plants with disease, or does it also detect germs and viruses in the environment? 

This is great for scanning a crowd and finding out who needs a Lesser Restoration spell. Not only can you detect everyone in range who is sick, but since it identifies the disease, you can use it to triage and see who needs healing most urgently. A Heal spell will cure one target of all the diseases it has. For those cases where an epidemic disease is already racing through the population, bring in that rare Cleric who can cast Mass Heal and cure all the diseases in everyone you can crowd within the range.

Since the spell doesn't specify that it detects diseases in creatures, I would say that it will detect diseased plants that don't qualify as creatures. However, I've been unable to find any way in the system to cure diseases in non-creature plants.

Alcohol counts as a poison, so the spell wouldn't do anything.

does the spell give the name of the disease?

One of the obvious use cases for this is to detect poison in food and drinks. Its irrelevant whether a DM decides alcohol is a poison or not because you detect the presence of all poisons and immediately identify the kind of poison, poisonous creature, or disease in each case. If a tankard of ale has been spiked with a poisonous substance, you will immediately know that there is more than just ale in that tankard.