I've honestly had better DNA extraction from frozen plant tissue than from fresh. But mine were stored in a "non-frost free" -20 freezer. That sort of freezer does NOT go through freeze & thaw cycles so it builds up a layer of frost over time. It's better for science since the temperature is more stable. Does it matter for the next step in your experiment if you have less DNA than expected?

I too had better results in frozen tissue rather than fresh one. Interference of phenolic compounds was also lower in frozen tissues.

@ Katie A Clark ! Precisely I detect Phytophthora DNA from the plant crown tissue. In high infection cases, I get amplification of Phytophthora DNA from tissue stored at -20 C (even after multiple thawings), while in cases of low plant infections the Phytophthora DNA disappears if plants are stored at -20 C. To some extent the DNA out put get reduced after freezing at -20 C.

Dear Mustafa,

No any difference in quantity and quality of DNA isolated from fresh and -20 stored plant samples. You have to check your DNA isolation protocol again.

Literature says that plants can only be store at -70 C or below for DNA extraction.  This means there is degradation of DNA at warmer temperatures like -20C. For instance following links of literature

See chapter 3 of the following book 

https://link.springer.com/content/pdf/10.1007%2F978-3-319-16259-1.pdf

Qiagen detailed recommendations ( QIAGEN News article 2004 e8) confirms that plant can only be store at -80 C while animal tissues can be stored at -20 C.

https://www.qiagen.com/at/resources/faq?id=6cb74cba-f16d-4069-92a0-68d904b2e615&lang=en