yídàn fāxiàn chónghài, jiù xiān kàn yèpiàn.

This is because of the common tone-change rule for 一.

• 一 is normally first tone: yī
• But before a fourth-tone syllable, it usually changes to second tone

Since 旦 is dàn (4th tone), 一旦 is pronounced yídàn.

This kind of change is very common:

• 一天 → yìtiān (before 1st tone, it often becomes 4th tone)
• 一个 → yíge (before 4th tone)

So yídàn is the natural pronunciation.

一旦...就... is a common pattern meaning something like:

• once... then...
• as soon as... then...
• sometimes if ever... then...

In this sentence:

• 一旦发现虫害，就先看叶片。

it means:

• Once pest damage is discovered, first look at the leaves.

The pattern shows a condition followed by the action that should happen immediately or naturally after it.

Structure:

• 一旦 + condition/event
• 就 + result/action

So here:

• 一旦发现虫害 = once you discover pest damage
• 就先看叶片 = then first check the leaves

就 helps connect the two parts and makes the relationship feel tighter and more immediate.

With 就:

• 一旦发现虫害，就先看叶片。
• Once pest damage is found, then first check the leaves.

Without 就:

• 一旦发现虫害，先看叶片。

This is still understandable, but 就 makes the sentence sound more natural and complete with the 一旦 structure. Native speakers often like the pairing 一旦...就....

So 就 is not always absolutely required for basic understanding, but it is very standard and idiomatic here.

先 means first.

So:

• 看叶片 = look at the leaves
• 先看叶片 = look at the leaves first

It tells you the order of actions. The sentence implies that when pest damage is noticed, the first step is to inspect the leaves.

In Mandarin, 先 usually goes before the verb:

• 先看 = first look
• 先说 = speak first
• 先吃饭 = eat first

Mandarin often leaves out the subject when it is obvious from context.

In a sentence like this, especially in instructions, manuals, or advice, the subject is usually understood as:

• you
• the person doing the inspection
• or someone in general

So the full idea is something like:

• Once you discover pest damage, first check the leaves.

Chinese often sounds more natural than English when it omits subjects that are easy to infer.

发现 means to discover, to detect, or to notice.

It is a bit more than simply seeing with your eyes. It often suggests that you become aware of something.

Compare:

• 看见 = to see
• 发现 = to discover / notice / detect

Examples:

• 我看见一只鸟。 = I saw a bird.
• 我发现叶子上有虫。 = I noticed there were insects on the leaves.

In this sentence, 发现虫害 means noticing or detecting pest damage, not just physically seeing something in a vague way.

虫害 is a fairly specific term, often used in agriculture, gardening, and plant care.

It means:

• damage caused by insects
• pest infestation
• insect pest problem

Breakdown:

• 虫 = insect / bug / worm
• 害 = harm / damage

So 虫害 is not just bugs themselves; it refers more to the harm or infestation caused by them.

That is why the sentence sounds like practical plant-care advice.

Both relate to leaves, but they are not exactly the same in tone or usage.

• 叶子 is the everyday word for leaf/leaves
• 叶片 is more technical, formal, or scientific, often used in biology, agriculture, and plant inspection

In a sentence about checking for pest damage, 叶片 sounds more precise and appropriate.

So:

• 看叶子 = everyday speech, very natural
• 看叶片 = more specialized or instructional

You can think of 叶片 as fitting the practical, observational style of the sentence.

Here it means the leaves / leaf surfaces / leaf blades in general, not necessarily just one single leaf.

Chinese nouns often do not mark singular vs. plural clearly unless the speaker wants to.

So:

• 叶片 can mean a leaf blade
• but in context it can also mean the leaves as a category

In this sentence, it most naturally means:

• check the leaves
• or inspect the leaf surfaces

The context, not the noun ending, tells you whether it is singular or plural.

Because 先 is an adverb, and in Chinese adverbs usually go before the verb they modify.

So the natural order is:

• 先看 = first look

Not:

• 看先

Basic pattern:

• 先 + verb

Examples:

• 先吃饭 = eat first
• 先回家 = go home first
• 先检查叶片 = inspect the leaves first

So 就先看叶片 is the normal word order:

• 就 = then
• 先 = first
• 看 = look at
• 叶片 = the leaves

It reads most naturally as an instruction or procedural recommendation.

Depending on context, it can be understood as:

• When pest damage is found, first check the leaves.
• If you discover pest damage, the first thing to do is check the leaves.

Chinese often gives instructions without using words like please, should, or you must, especially in manuals, guides, or technical advice.

So grammatically it is a statement, but functionally it works like an instruction.

The comma separates the two main parts of the sentence:

• 一旦发现虫害
• 就先看叶片

This matches the conditional structure:

• once/if...
• then...

Chinese uses commas quite often to make sentence structure clearer, even where English might or might not use one. Here it helps show the pause between the condition and the response.

Yes. That is a very natural translation.

Possible translations include:

• Once pest damage is discovered, first check the leaves.
• As soon as you notice pest damage, first inspect the leaves.
• If pest damage is found, look at the leaves first.

The exact English wording can vary, but the Chinese structure stays the same:

• trigger/condition: 一旦发现虫害
• next action: 就先看叶片

So as soon as is a perfectly reasonable way to capture the meaning.