Producing a good crop of tomatoes in the tropics and subtropics challenges the best of farmers. Fighting pests and diseases is a never-ending battle as pathogens evolve and develop resistance, rendering once-effective pesticides useless. In some cases the pathogens have the upper hand: For instance, pathogen pressure from Tomato yellow leaf curl virus (TYLCV) is so intense in some “hotspots” in India and West Africa that farmers have stopped producing tomatoes in the open field.
Tomato leaf curl viruses are caused by begomoviruses transmitted by the whitefly, Bemisia tabaci. Adult whiteflies often hide on the undersides of leaves and look like tiny white moths (1-2 mm in length). Cucumbers, cotton, eggplants, sweet potato, and some weeds are favorite whitefly hosts. Whiteflies feed on infected plants and then spread the virus to non-infected plants. Tomato plants infected in early growth stages (before flowering) are severely stunted with upright bushy shoots. Leaves are reduced in size, curled and puckered; they may also become yellow between veins and around the edges. The plant may be stunted, yield less, and cease growth.
Tomato leaf curl virus (ToLCV) and Tomato yellow leaf curl virus (TYLCV) are the most damaging of the tomato viruses; diseases caused by these viruses can wipe out an entire tomato crop if the infection occurs at an early stage. Farmers in the tropics and subtropics often misuse pesticides in an attempt to control the whitefly vector, and as a result B. tabaci has developed resistance to many insecticides. However, insecticides alone seldom prevent infection because it takes only a few whiteflies per plant to spread the viruses.
Resistance is the cheapest, simplest and most effective way to control tomato leaf curl viruses. Since the 1970s, breeders around the world have sought to develop resistant tomato cultivars through the introgression of resistance genes.
AVRDC - The World Vegetable Center initiated a program in the early 1990s to introduce ToLCV-resistance genes into tomatoes through conventional plant breeding techniques. With partners at the Natural Resources Institute, University of Greenwich, UK and the University of Agricultural Sciences, Bangalore, India, 3 ToLCV-resistant open-pollinated tomato varieties (‘Sankranthi,’ ‘Nandi’ and ‘Vybhav’) were developed and released officially in 2003-2004 in India. Public institutions and private seed companies obtained breeder’s seed of the high yielding, highly resistant varieties to multiply and distribute.
By 2007, more than a million farmers in India had adopted the improved varieties, and AVRDC had distributed the improved seed to 13 countries in Asia and Africa. In India, farmers that grew the resistant varieties realized an average profit per crop of US$320, and saw their incomes increase from US$768-2,195 to US$1,097-4,155 per hectare. Due to the widespread adoption of the resistant tomatoes, farmers reduced the number of pesticide sprays from 9-20 to 7-15 during the lifetime of a crop.
Tomato yellow leaf curl virus disease (TYLCVD) is caused by more than 50 different whitefly-transmitted begomoviruses–and for that reason it is one of the most difficult tomato diseases to control. In 2009, AVRDC breeders led by Dr. Peter Hanson had a breakthrough: Through “gene pyramiding”–combining multiple Ty genes into AVRDC lines–breeders developed tomatoes with resistance to several whitefly-transmitted begomoviruses that cause TYLCVD.
Gene pyramiding is a breeding technique used to introduce multiple genes into a plant, each of which imparts resistance to a specific pest or disease. Because a pest must overcome all of the resistance genes simultaneously to survive, it is more likely the vegetable line or variety will retain its resistance over a longer period–perhaps for several decades.
Before any pyramiding could be done, however, breeders around the world had to find the Ty genes by evaluating hundreds of tomato accessions. Researchers at the University of Florida USA (Ty-1, Ty-3), Hebrew University in Israel (Ty-1), and Hisar University in India (Ty-2) found the genes they were looking for in some accessions of the wild tomatoes Solanum habrochaites, S. chilense, and S. peruvianum. AVRDC obtained different lines with 1 or more Ty genes, and crossed its tomato lines with these sources. Using molecular markers, AVRDC breeders then selected lines carrying multiple resistance genes.
The improved lines carry various combinations of resistance to begomoviruses, bacterial wilt, fusarium wilt, and early blight, as well as good tolerance to heat. Interested farmers and partners can obtain seed of the improved fresh market and dual-purpose (fresh consumption and processing) lines through the AVRDC website.
Although resistance offers the best means to control TYLCV, providing resistance to 50 or more begomoviruses remains a challenge. The improved tomato lines are resistant to some, but not all, begomoviruses. AVRDC’s disease-resistant lines coupled with good agricultural practices such as the use of net houses or net shelters to exclude whitefly offer farmers an integrated strategy for safe tomato production. DFID provided support for this sound approach through its Tropical Whitefly Project. The judicious use of imidacloprids or other effective whitefly insecticides can lower whitefly numbers and help protect resistant varieties from infection.
Besides holding disease at bay, there’s an additional objective the Ty resistance varieties must conquer: Farmer approval. These new lines must satisfy yield and fruit quality requirements of farmers and markets. Multilocation trials are ongoing in Central America, East Africa, and India to solicit farmers’ impressions and comments about the resistant varieties, and to collect data on varietal preferences and characters for future breeding programs. From 2010 to date AVRDC has sent 1176 seed packets of the multiple Ty lines to 118 recipients in 46 countries.
The Ty resistance tomatoes are the latest in a very long line of improved tomato germplasm developed at AVRDC. Since 1978, 172 open-pollinated tomato varieties based on the Center’s lines have been released in 41 countries, offering farmers the opportunity to reduce their crop production costs by saving their own seed.