Plant Disease of the Week No. 13e (Blueberry rust)

Pathogen attributes: Naohidemyces vaccinii is heteroecious (having two hosts in its life cycle). Hemlock (Tsuga sp.) is the alternate host. The pathogen is also considered a "species complex" that occurs on many eriaceaous hosts, and having two forms in North America (an eastern form and a western form).

Hosts of Naohidemyces vaccinii: Hosts of the pathogen include Vaccinium spp. (including blueberry, cranberry and huckleberry, and V. reticulatum ('ohelo)), Gaylussacia (huckleberry), Tsuga (hemlock, hemlock spruce), Rhododendron (including azalea), Lyonia, Menziesia (mock azalea), Pernettya, Hugeria, Pieris, Leucothoe and Oxycoccus.

History in Hawaii: The pathogen was first collected by Stevens in Hawaii in 1921.

Geographic range of blueberry rust: the disease has been reported in Australia, Europe, Argentina, Asia, Mexico, Canada and the USA.

Disease cycle:

(1) in the Northern United States and Canada: The rust has a macrocyclic life cycle where hemlock (Tsuga sp.) exists, having the following different spore stages in its life cycle -

a) Airborne aeciospores infect young blueberry leaves in spring or early summer

b) Uredinial pustules develop on lower leaf surfaces of blueberry. Urediniospores form within these pustules. The urediniospores can continually re-infect the host throughout the season.

c) Telia form on lower surface of blueberry leaves late in the season (fall - winter).

d) In early spring, teliospores germinate, forming basidia and basidiosopres. Basidiospores are released from the blueberry leaves and infect hemlock needles. Pyncia develop on hemlock needles.

e) Aecia develop on hemlock needles (early summer). Aeciospores are released to infect blueberry, starting the yearly disease cycle over again.

(2) in Hawaii and the southern United States: The rust is presumed to be principally microcyclic, having in this case mainly one spore stage, the urediniospre. In other words the epidemics in Hawaii are driven exclusively by urediniospores. Although experimental plantings of two hemlock species were installed in the early 1930's on the island of Hawaii (Tsuga heterophylla (western hemlock) at Honualua Forest Reserve and Tsuga mertensiana (mountain hemlock) at Mauana Kea and Pohakuloa Forest Reserve), we do not know if these plants still survive today. Therefore, urediniospores probably drive the blueberry epidemics exclusively in Hawaii and the fungus over-seasons on living Vaccinium species.

Symptoms: About 10 days after inoculation, the disease appears first appears as tiny, chlorotic yellow spots on the upper leaf surface of young blueberry leaves. Spots later turn reddish brown and may be surrounded by a slight chlorotic yellow halo. Uredinia are produced on the lower surface of leaves, appearing as pustules which produce yellow-orange urediniospores. Yellowish orange rust pustules soon become visible scattered over the lower leaf surfaces. There may be anthocyanescence (reddening) of affected leaves. Leaves can turn brown, curl up and drop from heavily diseased plants.

Principal negative effects of blueberry rust: defoliation (direct effect), poor yields (indirect effect).

Integrated Management

1. Choice of blueberry variety

The best way to control blueberry rust is to plant a resistant variety, but we have not idenitified one yet. Unfortunately, the six southern highbush varieties are all rather susceptible to blueberry rust at two experimental locations (Lalamilo and Mealani), although they were less rusted at the warmer, drier location (Lalamilo, please see below).

Please refer to the following publication for information about the blueberry varieties in the USDA trials at Mealani:

Zee, F., Hummer, K., Nishijima, W., Kai, R., Strauss, A., Yamasaki, M., and Hamasaki, R. 2006. Preliminary yields of southern highbush blueberry in Waimea, Hawaii. University of Hawaii Cooperative Extension Service F&N-12.

Sources of resistance to blueberry rust do exist among northern highbush varieties, but many of these varieties probably are not well suited for most of Hawaii's tropical environments, save for some high-elevation locations. It is possible that though plant breeding a rust-resistant southern highbush blueberry variety suited for Hawaii could be developed.

An online Cornell University publication on blueberry rust indicates the following host reactions for northern highbush varieties (not yet evaluated in Hawaii):

Resistant: Bluecrop, Burlington, Collins, Dixi, Earliblue, Gem, Ivanhoe, Olympia, Stanley and Weymouth

Moderately susceptible: Jersey, Herbert, Berkeley, Blueray and Pacific

Susceptible: Coville, Pemberton, Washington, Altlantic

2. Choice of planting location

Choice of planting location could be a key to growing this crop efficiently in Hawaii. A location with a relatively cool and wet climate should probably be avoided.

The six southern highbush varieties also were planted at Lalamilo on the island of Hawaii (nearby to the Mealani Experiment Station in Waimea). In 2007, the blueberry rust epidemic at Lalamilo was markedly less severe than at the planting of the same varieties in Waimea (R. Hamasaki, personal communication).

Table 1. Comparison of average daily air temperatures and total yearly rainfall data for two locations where southern highbush blueberry varieties were grown by the USDA/CTAHR. The locations are the UH-CTAHR Lalamilo and Mealani Experiment Stations on the island of Hawaii. By 2007, a severe blueberry rust epidemic had developed. The blueberry rust epidemic at the cooler, wetter, Mealani (right) was reportedly much more severe than the blueberry rust epidemic at the warmer, drier Lalamilo station. Lalamilo represents a superior environment. Note: These two stations are very close to each other, only several miles apart.

		Lalamilo+			Mealani**
Year	T Max (F)	T Min (F)	Rainfall (in)	T Max (F)	T Min (F)	Rainfall (in)
1997	74.92	57.39	32.60	71.82	55.88	57.57
1998	72.88	55.84	32.09	68.93	54.83	72.36
1999	73.47	56.78	19.78	69.67	54.63	48.65
2000	73.60	56.79	16.63	71.65	54.28	37.41
2001	73.63	57.58	23.49	72.07	54.41	48.58
2002	74.00	58.08	33.71	73.25	54.64	68.51
2003	74.74	57.69	21.62	73.84	54.13	42.59
2004	75.72	59.09	49.41	74.97	54.93	83.09
2005	75.43	58.69	28.60	73.69	54.38	47.44
2006	75.23	58.72	22.60	73.11	54.42	51.60
2007	75.60	58.85	24.08	73.16	53.65	34.53
Average	74.48	57.77	27.69	72.38	54.56	53.85

+ less severe blueberry rust epidemic

** more severe blueberry rust epidemic

Table provided by Mary Kaheiki (UH-CTAHR).

Information on relative severity of blueberry rust epidemics was provided by Randall Hamasaki (UH-CTAHR).

3. Start with disease-free planting material

It is essential that new fields are not planted with rusted blueberry plants.

4. Use of fungicides

There are only a few fungicides available in Hawaii for blueberry rust management, and the data indicate so far that they are either not very effective in suppressing epidemics or there are phytotoxicity issues associated with their use.

5. Intercropping

Use polycropping systems that include non-hosts of the pathogen. Non-hosts can provide a physical barrier for spore dispersal among plants.

6. Do not purchase rusted plants from nurseries

New fields must be started with disease-free plants. Do not purchase or plant any blueberry plants that have rust symptoms.

7. Choice of production system

Our experience indicates that growing blueberries as a row crop in fields, out in the open, can lead to the development of severe epidemics that cannot be controlled effectively using existing fungicide or other management tactics. It is probably much better to grow blueberries in more controlled environments such as hoop houses or greenhouses, similar to the way many tomato farmers cultivate tomatoes in Hawaii.

8. Quarantine

Blueberry rust is regarded as a serious quarantine pest in some locations, for example in Australia. Because the blueberry rust pathogen is reported to be genetically variable worldwide, it might be important to prevent the introduction of new forms or strains of the pathogen into Hawaii on blueberry or other hosts.

Here is a quote from Australia's Division of Primary Industries:

"Blueberry rust ... has been reported in Europe, Argentina, Asia, Mexico, Canada and the USA.... In order to maintain Victoria's disease-free status, an order has been in effect since 2002, controlling the importation into Victoria of materials which may carry the disease. These materials include: plants - Vaccinium spp. (including blueberry, cranberry and huckleberry), Gaylussacia, hemlock, Rhododendron (including azalea), Lyonia, Menziesia, Pernettya, Hugeria, Pieris, Leucothoe , Oxycoccus, and any agricultural equipment or packaging used in the farming or subsequent processing of these plants." (Australia, Division of Primary Industries, click here for link.)

Fungicides:

In environments that are very conducive for blueberry rust disease, frequent applications of fungicides will be necessary to successfully grow monocrops of blueberries in Hawaii. Even with frequent fungicide sprays, this disease may still prove too difficult to manage. Also, there may be phytotoxicity of some fungicide products to blueberry leaves (test them first and avoid tank mixes of incompatible products).

Table 2. Fungicides registered for control of blueberry rust in Hawaii (click on product name to see the label as a .pdf file)*

Product name	Active ingredient(s)	Formulation
Cabrio	Pyraclostrobin (20%)	Water dispersable granules
Pristine	Pyraclostrobin (12.8%) Boscalid (25.2%)	Water dispersable granules
Tilt	Propiconazole (41.8%)	Emulisifiable concentrate

*source: HPIRS (Hawaii Pesticide Information Retrieval System) and Dr. Mike Kawate, UH-CTAHR.