Rosette is a nutritional deficiency disease caused by certain soil
conditions which make zinc unavailable to the pecan tree. All pecan
trees require zinc for growth.
Trees showing the first symptom of zinc deficiency have yellowed tops.
The individual leaflets when examined are yellowish and mottled. The
next season the foliage may be yellowish and the leaflets narrowed and
crinkled. More severely affected trees produce foliage which is a
yellowish to reddish-brown overall color, and the leaflets are very
narrow with reddish-brown spots and may be perforated. Shoots are much
shortened and the leaves are produced in compact bunches of dense
foliage.
Trees affected by rosette for several seasons have many dead shoots and
small branches from the dying-back of each season’s growth, Figure 4.
Such trees are greatly stunted, of poor vigor and produce few, if any,
nuts.
_Control._—Rosette is controlled readily by applying zinc sulfate to the
tree either as a foliage spray or in the dry form as a soil application.
Where a disease and insect spray control program is being carried out,
zinc sulfate may be added to the spray mixture.
Foliage spray. Two pounds zinc sulfate (36 percent) per 100 gallons of
water.
First application: after pollination when tips of nutlets turn brown.
Second application: 3 to 4 weeks later.
Third application: 3 to 4 weeks later.
Soil application. Application of zinc sulfate to the soil, particularly
in a large orchard is a more expensive operation, but it provides longer
protection against rosette.
In highly alkaline soils, or soils that readily fix zinc and make it
unavailable to the tree, foliage spray applications of zinc sulfate are
more economical because of the excessive rates required to supply
available zinc through the soil.
Rate of application of zinc sulfate: Mildly rosetted trees—apply 5
pounds zinc sulfate (36 percent) annually for 2 to 3 years. Severely
rosetted trees—apply 5 to 10 pounds zinc sulfate (36 percent) annually
until rosette symptoms disappear.
Time and method of application: Apply zinc sulfate to the soil around
trees in late February or early March. Broadcast zinc sulfate under the
tree from the trunk to several feet beyond the limb canopy. Disking,
harrowing, or any operation that mixes the zinc sulfate with the soil,
is desirable to prevent washing away and surface soil fixing of zinc.