The impact of invasive fungi on agricultural ecosystems in the United States

Invasive fungi and other non-indigenous plant pathogens have had a significant effect on American agriculture for hundreds of years. At present crop loss due to invasive plant pathogens, especially fungi, is estimated at $21 billion per year in the United States, greater than the loss caused by non-indigenous insects. Plant pathogenic fungi are difficult to detect and identify. Thus knowledge of which fungi pose a threat is essential to prevent their entry by means others than inspection. In this paper, examples are presented of invasive fungi on agricultural commodities introduced into the United States. In all cases two factors have been crucial: first, the pathway through which these fungi have entered, and second, systematic knowledge to prevent and respond to the new invasive species. Historically important plant pathogens such as black stem rust of wheat still cause considerable damage while others such as late blight of potato appear to be having a resurgence. Known previously in Australia, then moving to Africa and South America, the virulent species of soybean rust appeared in the U.S. in 2004 but has not been as devastating as anticipated. Plant pathogenic fungi on specialty crops such as daylily, gladiola and chrysanthemum are threatened by rust fungi recently found in the U.S. apparently brought in on infected germplasm. A crisis in the export of U.S. wheat occurred in the late 1990's when the molecular diagnostic test for Karnal bunt gave a false positive response to a closely related but previously unknown species. Many potentially dangerous plant pathogens of crop plants have not yet been introduced into the U.S. It is critical that meticulous surveillance be conducted as plant material enters the country as well as where crops are grown prior to shipment. In addition, the scientific infrastructure is needed to be able to respond quickly to new invasive fungi. This requires sound systematic knowledge of plant pathogenic fungi both in the U.S. and around the world and a cadre of systematic experts who can characterize invasive fungi.