Roses are popular garden and landscaping plants that usually grow well with only a little care. However, they are subject to some problems, including leaf drop. This may manifest as a browning or yellowing of the leaves, which then fall off, or as a sudden loss of apparently healthy leaves. Roses may lose their leaves due to environmental causes, disease or simply because of seasonal factors.
Heat and Drought
Roses may lose their leaves due to excessive heat, especially late in the season. According to My San Antonio, some varieties, like knock out roses, tend to suffer from brown and falling leaves in late summer. Leaf drop is most likely to occur in very hot, dry weather, when water evaporates from the plant faster than the roses can replace it. Irrigating problem roses can help, but may not completely correct the problem.
Black spot fungus attacks the leaves at the bottom of the rose bushes before moving upward. This fungal disease is recognisable by the round black or dark brown blemishes that form in the middle of the leaves. Leaves affected by black spot turn brown on the edges and eventually fall off. This fungus overwinters on fallen rose leaves; homeowners should rake and remove all diseased leaves in the fall to prevent future infestations. Other problem fungi include powdery mildew and rust. The University of Illinois recommends spraying affected roses with a fungicide on a weekly basis.
Container roses grown in poorly drained pots often lose their leaves due to excessive water collected around the roots. This problem may also occur in roses planted in clay soils or roses which receive too much water. Overwatered rose leaves turn yellow before falling off. Texas A&M University recommends installing tile drains or planting roses in raised beds to avoid drainage problems.
Roses drop their leaves due to insect infestations, including spider mite infestations. These tiny creatures create colonies underneath rose leaflets, where they drain the sap from the leaf. Leaves may initially look grey in the centre before falling off, although some leaflets turn yellow before dropping. According to the Tri-State Rose Society of Chattanooga, spraying leaflets with high-pressure water every two days can remove mite colonies. Gardeners may also treat spider mites with pesticides.