Plumeria turning black

[Than]

I have two plumeria cuttings. I put both in pure perlite over a heat mat and soon they both had long nice white roots. I repotted one to a new pot with a mixture of 50% perlite and 50% soil while the other one is still in pure perlite. The repotted one started having black spots in the middle of leaves which then curled and died. Today I noticed that the one which is in pure perlite also started doing the same. I water once a week the one in soil+perlite while the one in pure perlite I've only watered twice in a month. What could be the issue? I'm attaching photos. 

 

[Silas_Sancona]

8 minutes ago, ego said:

I have two plumeria cuttings. I put both in pure perlite over a heat mat and soon they both had long nice white roots. I repotted one to a new pot with a mixture of 50% perlite and 50% soil while the other one is still in pure perlite. The repotted one started having black spots in the middle of leaves which then curled and died. Today I noticed that the one which is in pure perlite also started doing the same. I water once a week the one in soil+perlite while the one in pure perlite I've only watered twice in a month. What could be the issue? I'm attaching photos. 

 

Could be heat.. that will burn young leaves.  As long as the stems feel solid, and not soft / mushy, ..or looks like it is shriveling,  they should be alright.  As a general rule, you only want to water cuttings once a month ..until there is a good flush of fully developed / opened leaves.  Don't repot / disturb at. all. until you start seeing roots coming out of the drain holes.  NO fertilizer until well established as well.

[Than]

But if they stay in pure perlite for too long how will they get nutrients? 

[Silas_Sancona]

1 minute ago, ego said:

But if they stay in pure perlite for too long how will they get nutrients? 

The stems store nutrients, much like Cacti and other CAM - type plants..  Before they've filled a pot w roots,  leafed out fully, they aren't going to need any extra food.. Once filled out and growing , then you start feeding.

Additionally, if cutting- propagated Plumeria try to produce inflos,, cut them off.. No energy is to go toward flower production that first year after being rooted.  Not if you want healthy plants anyway..