Witches Broom on Blue Palo Verde
Witches broom disease of blue palo verde has become very common in southern Arizona landscapes.
Witches broom disease of blue palo verde (Parkinsonia florida) has become very common in southern Arizona landscapes. Blue palo verde is a native and the state tree of Arizona. It is very popular in urban landscapes due to its striking green-colored trunk and branches, spectacular yellow flowers in spring, fast growth rate, and excellent performance in the arid climate. Brooms are characterized by clusters of dense, short branches with leaves and without thorns and can be found throughout the tree canopy. Some trees have no brooms when transplanted into the landscape and others are composed almost entirely of brooms, giving the canopy the appearance of a dense shrub.
Problems associated with witches broom disease of landscape trees are broom dieback, breaking of large brooms during storms, and structural issues caused by the removal of branches and the possibility for sunburn to occur on branches in the canopy that become exposed to direct sunlight. Witches broom disease of palo verde is almost exclusively found in blue palo verde, however, very few cases have been observed in the other palo verde species such as Foothills palo verde (Parkinsonia microphylla), palo brea or Sonoran palo verde (Parkinsonia praecox), the Desert Museum hybrid (Parkinsonia x Desert Museum) or other hybrids in the trade.
Witches broom symptoms on palo verde were first described over 50 years ago, but their cause was unknown. Our research results have identified the presence of a previously unknown multi-segmented, single-stranded RNA plant virus, in the genus Emaravirus, in shoots, leaves, flowers, seeds, and suckers from blue palo verde trees that were affected by broom. The presence of the single-stranded RNA virus was not detected in the same tissues tested from plants showing no symptoms of broom disease. This plant virus associated with broom-affected palo verde trees had not been discovered until now. About 60% of the viral genome sequence is similar to another plant virus named High Plains wheat mosaic virus. Other emaravirus species are known that infect European mountain ash (Sorbus aucuparia), fig (Ficus carica), pigeonpea, (Cajanus cajan), raspberry (Rubus spp.), and rose (Rosa multiflora). The symptoms characteristic of emaravirus infection in these species are blotching, chlorotic ringspots, mottling of leaves, smaller than normal or stunted leaves, and densely clustered, multiple shoots.
All emaraviruses discovered so far have been found to be transmitted by an eriophyid mite, making it the only known ‘biological’ vector. These arthropods are often host specific, meaning they populate only one genus or possibly a single species of plant. The presence of the eriophyid mite Aculus cercidi (Keifer, 1965) has been associated with broom symptoms in palo verde for some time, and it was suspected to cause feeding damage that resulted in the development of broom symptoms. However, until now, no causal relationship had been established between the eriophyid mite and the broom symptoms in palo verde tree. Based on several years of collecting insects and arthropods from broom infested and healthy, non-symptomatic trees, we found consistently high populations of A. cercidi infesting trees with broom symptoms, while mites were in low abundance or rarely found in apparently healthy, non-symptomatic trees. Confirming the presence of the emaravirus in mites collected from broom infected trees provided further evidence of mites as the vector in transmitting the witches broom disease. This strong association between the presence of the emaravirus in broom of blue palo verde trees that were heavily infested with the suspected eriophyid vector, the absence of both virus and vector on non- symptomatic trees, and the presence of the virus in the eriophyid mites, strongly support a causal relationship between emaravirus and the witches broom disease of blue palo verde.
Eriophyid mites are sometimes confused with spider mites. Eriophyid mites are wedge-shaped, very small, and require ~20x magnification hand lens to see on a plant and at least 80x magnification to observe the different life stages more closely under a microscope. Identification of eriophyid mite species requires an image taken with an
electron microscope and the assistance of a mite expert; mite samples we collected have been identified as A. cercidi using these tools.
There is no cure for witches’ broom in blue palo verde. Our current project goals are to assist nurseries in producing virus-free and broom-free blue palo verde trees to reduce the source of virus in trees that are transplanted into landscapes. We are testing whether seeds from blue palo verde that is collected from broom affected shoots will result in seedlings and trees that are infected with the virus. We are also testing whether the use of miticides to control eriophyid mites on tree seedlings will prevent witches broom disease in the nursery.
Cutting out the shoots with broom symptoms in small or larger trees has not been a consistent cure for witches’ broom, as new shoots with broom symptoms usually regrow within a short period of time. In some cases broom regrowth did not occur immediately, but emerged again later.
Non-symptomatic (right) and dense, broom affected (left) branches of blue palo verde
Dieback of broom on blue palo verde canopy.
Tree canopy almost entirely broom with a few non-symptomatic branches.
Recently, through research, testing procedures for the emaravirus have been refined and plant samples can be reliably tested for the presence or absence of the virus. The diagnostic test uses quantitative polymerase chain reaction amplification, which is extremely sensitive and allows for a rapid turn-round time. Asymptomatic blue palo verde plants that show no symptoms of witches broom can still carry the virus and if eriophyid mites feed on those plants, they can potentially transmit the disease to the next plant they feed on.
How do I get my palo verde tree tested?
- Fill out submission form, which includes instructions for collecting and sending sample submission.
- Free testing of palo verde samples is offered for wholesale nurseries or retail nurseries, which is funded by our current research grant.
- $25 per sample for all others, with payment due upon receiving the invoice.