Η σήψη των ριζών είναι σοβαρή.

Η is the feminine singular definite article in the nominative case, equivalent to the.

It appears because σήψη is:

• a feminine noun
• singular
• the subject of the sentence

So:

• η σήψη = the rot

In Greek, the definite article is used very regularly, often more often than in English.

σήψη is in the nominative singular, because it is the subject of the sentence.

The basic structure is:

• Η σήψη των ριζών = the subject
• είναι = is
• σοβαρή = serious

A good learner habit is to ask: Who or what is serious?
Answer: Η σήψη των ριζών.
That tells you this noun phrase is the subject, so σήψη is nominative.

Because των ριζών means of the roots, and Greek uses the genitive for of-relationships.

Here:

• οι ρίζες = the roots (nominative plural)
• τις ρίζες = the roots (accusative plural)
• των ριζών = of the roots (genitive plural)

So the phrase works like this:

• η σήψη = the rot
• των ριζών = of the roots

Together: η σήψη των ριζών = the rot of the roots

This is a very common Greek pattern:
noun + genitive = noun of noun

Because ρίζες is the nominative/accusative plural form, while ριζών is the genitive plural form.

The dictionary form is usually singular:

• η ρίζα = root

Its plural forms are:

• οι ρίζες = the roots
• των ριζών = of the roots

So the ending changes because the case changes. Greek nouns change form depending on their grammatical role.

είναι is the present-tense form of είμαι (to be) meaning is / are.

In this sentence, it links the subject with the adjective:

• Η σήψη των ριζών = subject
• είναι = is
• σοβαρή = serious

So it functions just like is in English.

A useful thing to know: in Modern Greek, είναι is used for both:

• he/she/it is
• they are

So the form itself does not change between 3rd person singular and plural.

Because Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

The noun σήψη is:

• feminine
• singular
• nominative

So the adjective must also be:

• feminine
• singular
• nominative

That is why you get:

• σοβαρός = masculine
• σοβαρή = feminine
• σοβαρό = neuter

Since σήψη is feminine, the correct form is σοβαρή.

It can often mean both, depending on context.

In medical, agricultural, or technical contexts, σοβαρή can mean:

• serious
• severe

So in a sentence about plant disease, English might translate it naturally as either:

• Root rot is serious
• Root rot is severe

The exact best translation depends on context, but the Greek grammar is the same.

σήψη is pronounced approximately SEE-psi, more accurately /ˈsipsi/.

A few helpful points:

• the stress is on the first syllable: σή-
• ψ is pronounced like ps in lapse
• the final η sounds like ee in Modern Greek

So:

• σήψη ≈ SIP-see

And the full sentence is approximately:

i SIP-see ton ri-ZON EE-ne so-va-REE

More carefully:

• Η = i
• σήψη = SIP-see
• των = ton
• ριζών = ree-ZON
• είναι = EE-ne
• σοβαρή = so-va-REE

The accent mark shows which syllable is stressed.

So:

• ριζών → stress on the last syllable
• σοβαρή → stress on the last syllable
• σήψη → stress on the first syllable

Stress matters in Greek pronunciation, and it is normally written, so learners should pay attention to it from the beginning.

Yes, Greek word order is more flexible than English because case endings show grammatical relationships.

The neutral, straightforward order here is:

• Η σήψη των ριζών είναι σοβαρή.

But Greek can sometimes move things around for emphasis, for example:

• Σοβαρή είναι η σήψη των ριζών.

That sounds more marked or emphatic, something like:

• It is serious, the root rot or
• The root rot is serious indeed

For normal statement style, the original order is the most natural.

Greek often uses the definite article in places where English may omit it.

So:

• Η σήψη των ριζών είναι σοβαρή.

can be translated naturally as:

• Root rot is serious

even though Greek literally includes the.

This is very common. Greek articles are used more systematically, and English translation does not always keep them.

Literally, it is of the roots.
But in natural English, we often turn that structure into an attributive noun:

• η σήψη των ριζών = literally the rot of the roots
• natural English: root rot

Greek commonly expresses this idea with a noun plus a genitive phrase, where English might prefer a compound noun.

So yes, των ριζών is grammatically a true genitive phrase, but in translation it may become a compound expression.

The main dictionary forms are:

• η σήψη = rot, decay
• η ρίζα = root
• είμαι = to be
• σοβαρός / σοβαρή / σοβαρό = serious

From those dictionary forms, the sentence uses:

• η σήψη → same form as the dictionary form
• των ριζών → genitive plural of ρίζα
• είναι → present tense of είμαι
• σοβαρή → feminine singular form of σοβαρός

It sounds natural and correct, and it fits especially well in a technical, medical, agricultural, or educational context.

Why?

• σήψη is a fairly standard word for rot/decay
• ριζών makes the phrase precise
• σοβαρή gives a formal-neutral evaluation

So it is not strange in everyday Greek, but it sounds especially at home in written or specialist language, such as plant care, farming, biology, or pathology.