Historically, September has been the worst month to own stocks. And this year was no different. The S&P 500 recorded a 5% decline; the Dow Jones Industrial Average shed 4%; and the Nasdaq Composite lost 6%.

^SPX Chart

^SPX data by YCharts

Yet, for long-term investors, declining stock values can offer an opportunity to accumulate shares on the cheap. So, are there any diamonds in the rough among the September stock lemons?

Let's have a look at the three worst-performing S&P 500 stocks to find out.

One bull and one bear figurine standing on a stock chart.

Image source: Getty Images.

FMC

First up is FMC (FMC -1.06%), a maker of insecticides, pesticides, and fungicides. The company saw its shares tumble 23% in September due to a slate of bad news.

Most notably, a short-seller published a report alleging that FMC misled investors regarding the prevalence of generic competition in certain key markets such as China, India, and Brazil. Moreover, the report notes that FMC's revenue has already declined 30% year over year as agricultural prices have tumbled, and that increased generic competition may capture even greater market share in the coming months.

For its part, FMC has pushed back, arguing that the report "contains quotes and information that have been taken out of context, are factually incorrect or do not reflect the multiple dimensions of the FMC diamide growth strategy."

At any rate, investors should exercise caution in this case. Perhaps letting the dust settle would be wise before getting involved.

FMC Chart

FMC data by YCharts

Dollar General

Second on the list is Dollar General (DG 1.90%). This bargain retailer has endured an ugly September that saw its stock drop 19%. 

A leading cause for that pullback was the company's atrocious second-quarter earnings report. Dollar General reported its fourth straight miss on earnings per share (EPS) and lowered its full-year forecasts for sales and profits for the second time in 2023.

In short, Dollar General is struggling in much the same way as other brick-and-mortar retailers are right now. Gross margins are under pressure, falling 126 basis points (or 1.26%) to 31%. A growing cause is inventory "shrink" -- another word for theft. The company noted it expects an additional $100 million loss due to theft through the end of 2023. The company also noted increased competition from Walmart as a potential headwind in the latter half of the year.

While some investors might be tempted to jump on board this potential turnaround stock, Dollar General still faces several difficult challenges in the next few quarters. With a potential recession on the horizon, investors might want to look elsewhere.

Align Technology

Last is Align Technology (ALGN 1.33%), the maker of orthodontic teeth-straightening appliances. While September was dreadful, Align has actually turned in a positive year-to-date return with shares up 33%.

What's more, Align didn't have any notable negative catalysts in September. Rather, it appears investors simply soured on the company due to broader market sentiment.

The company did announce the acquisition of Cubicure GmbH, a 3D systems company, for 79 million euros. But that purchase should simply help Align to scale production of its custom teeth-straightening aligners and dental appliances.

Looking ahead, analysts expect Align to generate $4.4 billion in sales next year, with EPS rising 18% to $10.40 per share. In short, the company's fundamentals still look sound, and September appears to have been a much-needed pullback for a stock that has turned in a red-hot 2023 thus far. 

While the stock and its expensive valuation (shares trade at a price-to-sales multiple of 5.8) isn't for everyone, growth-oriented investors would be smart to consider Align.