Genetically modified (GM) plants can be made resistant to disease through genetic engineering techniques. Basically, the scientists find genes that can help the plant combat a particular disease and they implant those genes into the plant that they want to cultivate.
Some plants are more resistant to certain diseases and other plants are less resistant. What scientists try to do is to isolate the genes that make a plant resistant to a given disease. They then try to take that gene and implant it into the genome of some other plant that is not resistant to the disease. If they are able to do this, the second plant will be resistant to the disease just like the first one is. This is not always possible, but it is the basic way in which scientists can create GM plants that are more resistant to disease than their non-GM versions.