Xylella fastidiosa, a bacterial plant pathogen, has become a growing concern for Delaware’s forests, landscape trees, and potentially, its agriculture. This post will walk you through what this bacterium is, the plant diseases it causes, and how it has already started affecting trees in Delaware. We’ll use solid research and clear examples from four major studies, all showing how this plant disease is more than just a theoretical threat, it's already here.

Xylella fastidiosa is a bacteria that lives in the xylem—the water-carrying tissue—of plants. It's not visible to the eye, but its effects are easy to spot once it infects a tree or shrub. It causes several diseases, including bacterial leaf scorch (BLS), phony peach disease, and Pierce’s disease of grapevines. It spreads through insect vectors, specifically xylem-feeding insects like leafhoppers and spittlebugs. These bugs feed on an infected plant, pick up the bacteria, and pass it along to the next plant they bite.

Plants and Trees at Risk

Over 30 plant families can be infected. In Delaware and nearby states, the most commonly affected trees include:

• Red oak

• Pin oak

• American elm

• Sycamore

• Red maple

• Sweetgum

Shrubs like wild grape, blackberry, Virginia creeper, and even weeds like dandelions have also been identified as hosts.

How the Disease Shows Up

Symptoms usually start in late summer and are often mistaken for drought stress:

• Leaf edges turn brown (scorch)

• Yellow bands form between healthy and dead parts of the leaf

• Leaves drop too early

• Branch dieback

• Tree decline over 5-7 years

Xylella fastidiosa in Delaware

Based on multiple peer-reviewed studies and official reports, Xylella fastidiosa has already been identified in Delaware, especially among urban oak trees. Here are the key findings from four important sources:

Study 1 – Sherald & Kostka (1992)

This early study identified oak leaf scorch in Delaware as a result of Xylella fastidiosa. Red, black, and pin oaks were specifically mentioned. It also noted that the symptoms began in late summer and that the disease persisted for years.

Study 2 – Gregory & Pollok (2025)

This report from the University of Delaware Cooperative Extension confirmed BLS in trees across Delaware since the early 1990s. Testing is available through their diagnostic clinic for a small fee, supporting local monitoring and diagnosis.

Study 3 – Sherald (2007)

This research highlighted that Delaware shares similar environmental and ecological risks as neighboring states like Pennsylvania and New Jersey, where BLS is well established. It emphasized that Delaware's oak, elm, and sycamore trees are highly likely to be affected.

Study 4 – Potter (2017), USDA Forest Service

The USDA report on Delaware's forests confirmed bacterial leaf scorch as a growing issue. Red oak had a 2.1% mortality rate due to the disease—double the state average. Sweetgum was the second most impacted.

Summary Table: Presence and Impact of Xylella fastidiosa in Delaware

Study	Author(s)	Year	Confirmed Presence in DE	Affected Trees	Notable Findings
Study 1	Sherald & Kostka	1992	Yes	Red oak, pin oak	Early signs of BLS in urban trees
Study 2	Gregory & Pollok	2025	Yes	Oaks, maples	University of Delaware offers testing
Study 3	Sherald	2007	Likely	Oaks, sycamore, elms	Shared risks with nearby states
Study 4	Potter (USDA)	2017	Yes	Red oak, sweetgum	Mortality rates due to BLS increasing

How it Spreads in Delaware

The primary method of transmission in Delaware is through local leafhoppers and spittlebugs. The report from Sherald (2007) mentions potential alternative hosts like wild grapevines and blackberry in riparian zones, areas near water, which are common in Delaware.

These insects and wild hosts make controlling the disease difficult. Trees in urban and suburban areas are at higher risk due to closer proximity and higher planting density.

Managing the Disease

Right now, there’s no cure for Xylella fastidiosa. But several practices can help manage or slow down its spread:

• Remove infected trees when symptoms worsen.

• Test trees regularly in late summer or early fall.

• Avoid planting monocultures. A mix of species is harder for the bacteria to take over.

• Use less-susceptible trees like white oak or willow oak in urban planning.

• Keep trees healthy through watering and mulching.

• Prune affected branches if symptoms are limited.

• Chemical options like oxytetracycline have been tried but aren't fully effective.

Why Delaware Needs to Take This Seriously

Forests in Delaware cover about 355,000 acres, with red oak making up a significant portion. These trees are not just beautiful, they provide shade, oxygen, and even help cool down cities. Losing them to BLS would be a big environmental and financial loss.

Annual mortality rates for red oak (2.1%) and sweetgum (1.2%) are already higher than the state average. The risk is especially high in urban areas where many of these trees are planted as part of city planning.

Final Thoughts

Xylella fastidiosa is not just a southern or California problem, it’s in Delaware. Oak trees in your neighborhood or city parks could already be infected. We need more monitoring, education, and early management to stop it from spreading further.

Local homeowners, city planners, and forest managers must stay alert. By identifying infected trees early and removing alternative hosts, we can help protect Delaware’s natural beauty and forest health.

References:

• Sherald, J. L., & Kostka, S. J. 1992. "Bacterial leaf scorch of landscape trees caused by Xylella fastidiosa." Journal of Arboriculture 18(2): 57–63.

• Gregory, N. F., and Pollok, J. 2025. "Bacterial Leaf Scorch." University of Delaware Cooperative Extension.

• Sherald, J.L. 2007. "Bacterial Leaf Scorch of Landscape Trees: What We Know and What We Do Not Know." Arboriculture & Urban Forestry 33(6): 376–385.

• Potter, Stephen. 2017. Forests of Delaware, 2016. Resource Update FS-143. U.S. Department of Agriculture, Forest Service, Northern Research Station. https://doi.org/10.2737/FS-RU-143