Tropical race 4 (TR4), a reinvigorated strain of Panama disease, was first discovered in 1993. This virulent form of fusarium wilt destroyed Cavendish in several southeast Asian countries and spread to Australia and India.[102] As the soil-based fungi can easily be carried on boots, clothing, or tools, the wilt spread to the Americas despite years of preventive efforts.[102] Cavendish is highly susceptible to TR4, and over time, Cavendish is endangered for commercial production by this disease.[103] The only known defense to TR4 is genetic resistance.[102] This is conferred either by RGA2, a gene isolated from a TR4-resistant diploid banana, or by the nematode-derived Ced9.[104][105] Experts state the need to enrich banana biodiversity by producing diverse new banana varieties, not just having a focus on the Cavendish.