Plant diseases are caused by several factors, including bacteria, fungi, viruses, mollicutes and nematodes. Fungi are spore-bearing organisms that produce enzymes that break down plant or animal produces in order to absorb the nutrients. Fungal diseases have included Dutch elm diseases, apple scab and wheat stem rust. A fungal disease is what caused the Irish potato famine. "Late blight," another type of blight caused by Phytophthora infestans, has become the largest biological deterrent in global food production and has been the biggest reason for the use of pesticides worldwide. Bacteria is another disease-causing culprit and is often very easily spread, making it very contagious and infectious. Bacterial diseases can be spread by wind, water and through insects.