Final answer:
Oomycetes, a group of protists, are responsible for diseases such as potato late blight and downy mildew in crops like potatoes and grapes, significantly affecting agriculture.
Step-by-step explanation:
The group of protists that includes members which infect plants and destroy important crops such as potatoes, corn, grapes, and lettuce is known as oomycetes. These water molds are notorious for their devastating effect on agriculture. A famous oomycete, Phytophthora infestans, causes the dreaded potato late blight, which historically led to the Irish potato famine. Another oomycete, Plasmopara viticola, infects grape plants and causes downy mildew, which almost crashed the French wine industry in the 19th century. These plant parasites are responsible for significant crop losses and continue to impact farming practices and food security today.