A New Problem In Connecticut Beginning in the 1990s

Powdery mildew of tomato, caused by the fungus Oidium lycopersicum, is a relatively new disease in North America that has the potential to affect both field and greenhouse tomato production. Symptoms include white superficial mycelium on leaves and stems, yellowing, desiccation, necrosis and defoliation. A number of fungicides have been effective in controlling the disease, and while most commercial tomato cultivars are susceptible, two adapted cultivars and a breeding line are resistant to powdery mildew under field and greenhouse conditions.

Tomato powdery mildew may be caused by three pathogens worldwide. Leveillula taurica (Oidiopsis taurica) is a pathogen of a wide range of host species in warm arid to semiarid climates in Asia, the Mediterranean, Africa, and more recently the southwest United States. Erysiphe orontii ( E. cichoracearum and E. polyphaga) is another species common to many host plants in both temperate and tropical regions. However, the recent disease outbreak reported in eastern North America was due to a distinct fungus, Odium lycopersicum, based on characteristics including appressoria shape, conidia and conidiophore morphology.

Tomato plants infected with O. lycopersicum were submitted to the Connecticut Agricultural Experiment Station Plant Disease and Information Office for diagnosis in 1995 (greenhouse samples), 1996, 1997, and 1998 (greenhouse and field samples). Disease severity in greenhouse-grown tomatoes ranged from light to severe (up to 90% of foliage affected in nearly 100% of plants). The effects on yield were not determined. Symptoms included white superficial mycelium on leaves and stems, often with yellow margins, followed by desiccation, necrosis, and defoliation. In 1996 and 1997, field grown tomatoes were severely affected by the disease. The majority of reports were from home gardens, but commercial production fields were affected in 1998. At least two commercial tomato fields experienced losses despite fungicide use, especially in the inner canopy of late-season tomato plantings.

While the pathogen was first described in Australia in 1888, the distribution of O. lycopersicum has recently been increasing in Europe and in North America. Researchers from the UK reported a tomato powdery mildew with similar characteristics and host range to our isolates. A morphologically similar powdery mildew disease has been reported from Quebec and New York. Reports of an Erysiphe sp. from New Jersey and Florida may or may not be similar to the Connecticut isolates, as that fungus was reported to produce conidia in chains. An Erysiphe sp. from Hungary produced conidia in chains and did not infect tobacco, different from the reported host range of O. lycopersicum from England and from our findings in Connecticut. Additionally, we confirmed infection and symptom development on potato, tobacco, and eggplant under greenhouse conditions, in agreement with previous researchers and the first report of a powdery mildew pathogen on potato in North America.

O. lycopersicum has a remarkably wide host range of at least 13 plant families including the Cucurbitaceae and Solanaceae. We confirmed reports that the same isolate may produce a greater number of conidia on tobacco than on tomato. Plant age and/or environmental conditions may also affect host range, however, as we only observed infection on transplant sized plants of eggplant and eastern black nightshade in the greenhouse and not on mature plants under field conditions. The fact that host and/or environment may affect infection and the number of conidia borne per conidiophore may help explain the discrepancies in the literature cited above concerning Oidium on tomato.

This fungus was first described over a century ago and appears to be widely distributed. However, overwintering cleistothecia of this parasite have yet to be found or induced. The fungus may be heterothallic with only one mating type widely distributed. The teleomorph may not exist or may be present only in limited distribution. Researchers have hypothesized that the recent expansion in O. lycopersicum distribution may be due to a recent development of a type aggressive to tomato that has rapidly spread around the world.

The increasing distribution of the pathogen and the severity of tomato powdery mildew on greenhouse-grown or field-grown plants under warm, humid conditions will require the development and evaluation of control tactics. In general, commercial fields receiving fungicides have had reduced incidence and severity compared to unsprayed garden plots, but the disease is of concern to growers in IPM programs. Because disease severity has been greater under greenhouse conditions, the development of control tactics for use in greenhouses is particularly important.

In 1998 we evaluated the efficacy of five fungicides for control of powdery mildew under field conditions in Windsor, CT. On 2 Jun, 1998, five plants of Celebrity tomato were transplanted per plot to each of 5 replicate field plots in 3 rows 5 ft apart with 2 ft between plants in each plot. Two additional untreated border rows were planted to act as a source of inoculum. Plants were staked and tied but not pruned or suckered. Border plants were sprayed with a suspension of 1.6 x106 conidia per plant on 13 Jul. Plots were irrigated with 0.5 in. water on 15 Jul and 3 Aug. Tomato plots were left unsprayed or treated with either sulfur (Kumulus®, 15 lb/A), benomyl (Benlate®, 12 oz/A – no longer labeled for mildew on tomatoes), chlorothalonil (Bravo®, 1.5 lb/A), copper hydroxide (Kocide®, 2 lb/A), or azoxystrobin (Quadris®, 5.6 fl. oz/A) on 24 Jul, 7 Aug, 20 Aug, and 4 Sep. Plants received 50 gal water/A for complete coverage on the first spray date and 100 gal water/A at later dates. Plants were evaluated for percent foliage with powdery mildew on 10 Aug, 31 Aug, and 10 Sep. Ripening tomatoes were harvested from plots on 25 Aug, 3 Sep. 10 Sep, and 17 Sep. Data were analyzed by the nonparametric Kruskal-Wallis analysis at P=0.05.

Severe powdery mildew developed in the border plants and spread to the field plots (Table 1). Other foliar diseases, such as Septoria and Alternaria leaf spots, were low in incidence and not seen in the field plots or borders until late in the season. All fungicides evaluated resulted in less powdery mildew than developed in untreated control plots. Sulfur and Benlate® had superior efficacy early in the season, and Benlate® and Quadris® were superior at the late evaluation. Bravo® and Kocide® were intermediate in efficacy. While the trend was for lower yield in unsprayed plots, there were no significant differences in fruit yield as a result of the late season powdery mildew that developed in these experiments.

Tomato cultivar response to powdery mildew was evaluated in 1998 in field plots and greenhouse pots in Windsor. Twelve tomato cultivars were seeded 22 Apr and transplanted to 4-inch pots for greenhouse evaluations or to the field on 2 Jun. Ten replicate pots of each cultivar were kept on a greenhouse bench and naturally infected with powdery mildew. Greenhouse-grown plants were rated for percent foliage with powdery mildew on 14 Jul. Ten replicate plants of each cultivar were transplanted to field soils in 2 rows 39 inches apart with 36 inches between plants in each row. Plants were staked and tied but not pruned or suckered. No fungicides were applied. Tomato plants with severe powdery mildew were transplanted to border areas to act as a source of inoculum. Plots were irrigated with 0.5 in. water on 15 Jul and 3 Aug. Field-grown plants were evaluated for percent foliage with powdery mildew on 30 Jul, 10 Aug, 31 Aug, and 10 Sep. Data were analyzed by the nonparametric Kruskal-Wallis analysis at P=0.05.

There was severe powdery mildew present in both the greenhouse and field experiments (Table 2). Powdery mildew occurred earlier and was more severe under greenhouse conditions. Most of the commercial cultivars evaluated were very susceptible to powdery mildew. Differences in disease severity rank were apparent between 30 Jul and 10 Sep. Cultivars that had little powdery mildew early, such as Heinz 1439, Roma, and Rutgers, were severely affected later, underscoring the need to evaluate disease severity at different times during the season. Four cultivars, entries or breeding lines, Grace, DRW 5007, Hirol 3-21, and Lycopersicon hirsutum PI 247087 exhibited almost complete resistance to Oidium lycopersicum under both greenhouse and field conditions. Grace and DRW 5007 have been developed by DeRuiter Seeds for resistance to powdery mildew. L. hirsutum P1247087 has demonstrated near complete resistance in France and in Connecticut. Hirol 3-21 is a breeding line developed by crossing tomato to P1247087 and backcrossing 3 times to tomato. This resistance appears to be oligogenic with incomplete dominance. Hirol 3-21 is poorly adapted and did not produce marketable fruit. However, Grace and DRW 5007 are well adapted to Connecticut and produced marketable fruit under both field and greenhouse conditions.

Table 1. The effect of fungicide application on mean powdery mildew disease severity (percent leaf coverage) and tomato fruit yield in field plots, 1998.
Treatment	Rate	10 Aug	31 Aug	10 Sep	Fruit number	Fruit wt (lb)
Control		34.8 d*	80.6 c	82.0 c	266.0	89.7
Sulfur	15 lb/A	2.2 a	14.4 a	46.8 b	272.8	94.9
Benlate	12 oz/A	5.2 a	14.4 a	17.6 a	275.0	98.9
Bravo	1.5 lb/A	17.9 c	39.0 b	41.4 b	273.2	102.5
Kocide	2 lb/A	17.8 c	41.6 b	45.6 b	275.2	104.6
Quadris	5.6floz/A	10.4 b	16.2 a	23.2 a	280.8	106.6
P=		0.001	0.001	0.001	NS	NS
* Values in columns followed by different letters are significantly different according to the Kruskal-Wallis Multiple Comparison Z-Value Test at P= 0.05.

Table 2. Powdery mildew severity (percent leaf coverage) on selected tomato cultivars under greenhouse and field conditions, 1998.
Cultivar	14 Jul greenhouse	30 Jul Field	10 Aug Field	31 Aug Field	10 Sep Field
Baxter’s Early Bush	46.9 cd*	27.5 e	56.5 e	97.0 d	97.6 f
Better Boy	51.9 cd	4.3 bc	38.5 cd	80.0 b	74.0 bcd
Celebrity	59.3 d	6.5 bcd	31.0 bcd	87.5 bc	88.5 def
DRW 5007	2.5 a	0.0 a	0.0 a	0.0 a	0.0 a
Grace	1.9 a	0.0 a	0.0 a	0.0 a	0.0 a
Heinz 1439	53.8 cd	2.6 b	35.5 cd	94.5 cd	95.4 f
Hirol 3-21	6.3 a	0.0 a	0.0 a	0.0 a	11.7 ab
L. hirsutum	1.9 a	0.0 a	0.0 a	0.0 a	0.0 a
Roma	38.8 bc	3.3 bc	6.4 ab	78.0 b	72.0 bcd
Rutgers	48.1 cd	4.0 bc	38.0 cd	90.5 bcd	92.9 ef
Super San Marzano	20.6 ab	8.2 cd	21.5 bc	77.0 b	79.0 cde
Ultra Girl	47.5 cd	14.5 d	45.0 de	78.5 b	65.5 bc
* Values in columns followed by different letters are significantly different according to the Kruskal-Wallis Multiple Comparison Z-Value Test at P= 0.05.

For the latest powdery mildew management strategies, refer to the New England Vegetable Management Guide.

By: Dr. James A. LaMondia, Dept. of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station, Valley Laboratory, 153 Cook Hill Rd.

Windsor, CT 06095, Phone: (860) 683-4982, E-mail: lamondia@caes.state.ct.us

Published: Proceedings. 1999. New England Vegetable and Berry Growers Conference and Trade Show, Sturbridge, MA. p. 326-330.

Reviewed by: T. Jude Boucher, IPM, University of Connecticut. 2012

References:

• 1998-1999 New England Vegetable Management Guide. D. N. Ferro, ed. University of Massachusetts, Amherst.
• Belanger, R. R. 1994. Occurrence of powdery mildew (Erysiphe sp.) on greenhouse tomatoes in Canada. Plant Disease 78:640.
• Fletcher, J. T., and Smewin, B. J. 1988. Tomato powdery mildew. Plant Pathology 37:594-598.
• Karasevicz, D. M., and Zitter. T. A. 1996. Powdery mildew occurrence on greenhouse tomato plants in New York. Plant Disease 80:79.
• LaMondia, J. A., Smith, V. L., and Douglas, S. M. 1998. Host Range of Oidium lycopersicum on Selected Solanaceous Species in Connecticut. Plant Disease 83:34 1-344.
• Whipps, J. M., Budge, S. P., and Fenton, J. 5. 1998. Characteristics and host range of tomato powdery mildew. Plant Pathology 47:36-48.

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