BROOMING DISEASE OF WALNUT (BDW): Virus-like disease

DISTRIBUTION: TX, OK, AL, and other southeastern states. The AZ Department of Agriculture quarantine covers the states east of the eastern borders of ID, UT, and AZ.

DESCRIPTION: The causative organism is virus-like, possibly a mycoplasma (MLO Fig 1; 0.3-1.2 µm ). Graft transmission has been demonstrated. There is indication BDW may also be spread by leafhoppers. This organism lives in the inner bark’s food conducting cells. BDW is also called bunching disease.

HOSTS: Walnuts, pecans, and hickories. Heartnuts, Japanese walnuts and butternuts are particularly susceptible.

DAMAGE: Diseased trees have lower yields and inferior nut quality. The characteristic symptom is “brooms” of growth formed at terminals and suckers. Brooms develop from diseased branches and form tufted masses of thin, wiry shoots with abnormally short internodes (distance between places on the branch where leaves and buds are attached) and small crowded leaves. Dieback may occur with severe infections.

Infected branches frequently start growth earlier than normal in the spring and grow longer into the fall. This late fall growth retards normal cold-hardiness development, and the tips of infected branches are winter killed. Affected branches do not produce normal nut crops.

INSPECTION TIPS: In winter, look for a proliferation of stem shoots on large scaffold limbs (Fig 2).

In summer, look for characteristic “brooming” (Fig 3). This can sometimes be mistaken for Zinc deficiency. Leaflets with zinc deficiency are narrower, thicker, and more brittle than normal. Chlorosis (yellowing) generally develops between veins. In brooming disease leaflets tend to be wider, thinner, and softer than normal. They develop general chlorosis, not just confined between veins. Death of masses of main terminals is not a characteristic of bunch disease. Instead, dieback is general, not confined to terminals. Brooming often becomes visible after pruning or wind damage.

LIFE CYCLE: The life cycle of the causative organism is unknown. It is slow acting in walnuts, often infecting only one branch and requiring several years to move through the sap stream to other parts of the tree. Other trees are infected more rapidly.

References