Dr. Kumar, wilting of Guava or Lemon was caused by Fusarium sp. may be oxysporum,solani, chalamydosporum or proliferatum. Fusarium wilt is a soil-borne pathogen enter through the roots and interfere with the water conducting vessels of the plants. Prochloraz and Bromuconazole were the most effective fungicides against them. The effective concentration may vary between 0.1-2 microgram per mL depends on the severity. Regarding the resistant stock, I know, cultivar Allahabad Safeda of Guava is a Fusarium resistant. But I never come across any Fusarium resistant Lemon cultivar.