Large, tangy citrus fruits make grapefruit trees popular home garden additions in the hot summer and mild winter climates that the plants prefer. Like other citrus trees, grapefruits have white spring flowers and glossy, green leaves. But, also like other citrus trees, they are also susceptible to damage from several parasites. Learning to recognize and treat the infestations will keep your supply of refreshing fruit and juice intact.
Geography and Parasites
Grapefruit trees arrived in Florida from the West Indies in 1823. They are under commercial cultivation in Florida, Texas, California and Arizona. Grapefruit parasites vary according to location. Some parasitic bacteria devastating to citrus production in humid Florida does not affect crops in dry Arizona.
The most serious viral parasite is citrus tristeza (CTV). This stubborn disease results from a germ. Two phytophthera fungi attack grapefruit tree roots, as do microscopic worms called nematodes.
Common fungal diseases are phytophthera foot rot, Hendersonula branch wilt--or sooty canker--and Rio Grande gummosis, advise plant pathology specialist Mary Olson and colleagues at the University of Arizona Cooperative Extension. Aphids have transmitted the parasitic citrus trsiteza virus to grapefruit trees. Citrus stubborn disease is an infection of the bacterial mycoplasma Spiroplasma citri. Citrus nematodes attack roots while burrowing nematodes cause general tree decline.
Grapefruit foot rot's most serious symptoms occur underground. Vertical cracks in loose, waterlogged bark release a gummy substance. Rio Grande gummosis causes weeping, cracked bark and gummy trunk blisters. Sooty, black spores beneath the bark are the classic sign of Hendersonula-infected trees.
CTV causes small fruit with pitted stems and curling, yellowed leaves. Stunted, flat-topped trees with twig dieback, mottled, curled stiff foliage and small, acorn-shaped fruit indicate stubborn disease. Nematodes cause reduced fruit production, wilting and generally diminished vigor, says Texas A&M Extension plant pathologist Jose M. Amador.
Fungal Disease Prevention
Grafting trees to foot rot-resistant rootstocks, adequate soil drainage and moderate irrigation that doesn't saturate the roots are the best precautions against phytophthora infestations.
Cutting back small infected branches immediately--and larger ones when trees are dormant--is the recommendation for Hendersonula-infested trees. Prune affected branches back at least 12 inches past the canker. Use a 1 to 9 ratio bleach and water solution to wash the cut areas and cutting tools. Discourage cankers by painting spring-pruned branches with copper fungicide.
Immediate treatment of injured or weak trees is the accepted approach to Rio Grande gummosis prevention. Avoid planting the trees in poorly drained, high-saline soils or areas where freezing is common.
Non-Fungal Disease Prevention
Stubborn disease prevention includes propagating new trees from disease-resistant budwood and replacing affected trees with disease-free ones. Inoculating budstock used for propagation with a mild strain of CTV, and spraying young trees with an insecticide to kill the aphids that commonly transmit the virus can prevent severe outbreaks. A pre-planting application of nematicide to soil containing the worms is the suggested nematode prevention, according to Amador.