Report to the North American Strawberry Growers Association,2005
Title:COMPARISON OF PHOSPHOROUS ACID PRODUCTS FOR CONTROL OF LEATHER ROT AND OTHER STRAWBERRY DISEASES
Researcher
Dr. Annemiek Schilder, Department of Plant Pathology
104 Center for Integrated Plant Systems, MichiganStateUniversity, East LansingMI48824
Phone: 517-355-0483, Fax: 517-353-5598, Email:
Rationale
In the Midwest and northeastern US, strawberries are typically produced in perennial, matted-row plantings. Most of the fruit is sold fresh at local or regional farm markets and through pick-your-own operations. In Michigan, strawberries were produced on about 1,300 acres with a total market value of $63 million in 2003 (Kleweno & Matthews, 2004). For strawberry fruit to be sold fresh, it has to be of high quality. Numerous diseases can affect strawberry fruit qualityin these areas (Maas, 1998). In Michigan, common fruit rots include leather rot (Phytophthora cactorum), gray mold (Botrytis cinerea), anthracnose fruit rot (Colletotrichum spp.), and various minor fruit rots (Phomopsis obscurans, Rhizopus sp., etc.)(Maas, 1988). Leather rot is particularly problematic in wet years and sites with no or little straw cover. This disease imparts an objectionable odor and flavor to the fruit (Maas, 1998). A number of foliar diseases can also reduce plant vigor and fruit yield and quality. Among them are common leaf spot (Mycosphaerella fragariae), leaf blight (Phomopsis obscurans), scorch (Diplocarpon earliana), and angular leaf spot (Xanthomonas fragariae). The latter two can also cause necrosis of the calyx, thereby reducing fruit quality (Maas, 1998).
Aliette (fosetyl-Al) is a highly systemic fungicide which is labeled for control of leather rot and other Phytophthora diseases in strawberries. While this product is quite effective, it is rather expensive. More recently, several products with a similar active ingredient (phosphorous acid) have come on the market. Examples are Phostrol (Nufarm, Inc.), Agri-Fos (AgriChem, Inc.) and ProPhyt (Helena Chemical). Phostrol and Agri-Fos are already labeled for strawberries for control of leather rot and red stele. However, data on comparative efficacy and cost are hard to find. In general, these products are less expensive than Aliette and do not contain aluminum. Instead they contain potassium (Agri-Fos and ProPhyt) or sodium/potassium/ammonium (Phostrol) salts of phosphorous acid. Potassium salts supposedly have a lower risk of phytotoxicity than ammonium and sodium salts, so risk of phytotoxicity may vary between products. They have low mammalian toxicity, 0-day pre-harvest intervals, and also have foliar fertilizer properties.
The mode of action of these products is still largely unknown, but appears to be based on direct effects on pathogens as well as stimulation of natural defenses in the plant itself. These fungicides are all highly systemic, moving rapidly up and down in the plant, controlling both foliar and root diseases. While they tend to be very effective against Oomycetes (Phytophthora, Pythium spp., downy mildews), we have found in efficacy trials in other small fruit crops that they are also effective against other species of fungi, notably Colletotrichum and Phomopsis. We therefore expect these fungicides to be more broad-spectrum than their labels indicate. More information is needed on the efficacy of these products to be able to make recommendations to growers. For this project, we propose to comparethe efficacy of above-mentioned phosphorous acid products to each other and to Aliette and a standard fungicide program for control of leather rot and other diseases of strawberries.
Objectives
The specific objectives were to:
1)Compare the efficacy of ProPhyt, Agri-Fos, Phostrol and Aliette for control of foliar and fruit diseases
2)Assess potential phytotoxicity and estimate cost per acre per season for each product
Materials and Methods
The experiment was conducted in a commercial, matted-row strawberry planting of cv. Red Chief in Onondaga, MI. Rows were spaced 42 in. apart. Treatments were applied to 10-ft sections of row and were replicated four times in a randomized complete block design with no buffer rows. Straw was removed from the plots as much as possible to increase leather rot pressure. Sprays were applied with an R & D Research CO2cart-styled sprayer equipped with six bottles (0.8 gal each), a twin gauge Norgren pressure regulator set at 55 psi, and a single XR TeeJet 8002VS nozzle on a 5-ft spray boom. Spray volume was 100 gpa. Spray dates and corresponding phenological stages were as follows: 3 May (green foliage), 24 May (bloom), 2 Jun (bloom + green fruit), 14 Jun (green and red fruit, 1 day pre-harvest) and 21 Jun (red fruit, 1 day pre-harvest). Rainfall between sprays was 1.29, 0.0, 3.35, and 0.31 in., respectively. On 15 and 22 Jun, all ripe berries and leather rot infected fruit clusters were hand-harvested from the center 3 ft in each plot. Only the number of necrotic fruit clusters from 22 Jun are reported. Disposable gloves were used to pick berries and changed between plots to reduce cross-contamination. Ten berries from each plot were randomly evaluated for berry cap necrosis. To evaluate post harvest diseases, twenty-five berries from each plot were placed equidistantly on metal screens in aluminum trays and incubated at 72ºF and 100% RH. After 7 days, berries were visually assessed for fungal sporulation. The foliage was rated for diseases on 29 Jun. Disease severity was assessed as the percentage of the leaf area affected in the center 3 ft of each plot.
Results
Leather rot, berry cap necrosis, and post-harvest Botrytis gray mold pressure were moderate. Berry cap necrosis included browning caused by the leaf scorch fungus but some blackening caused by angular leaf spot bacteria. Foliar disease pressure was low due to the warm, dry growing season. The plot was irrigated, so there was sufficient moisture to support some disease development. All treatments significantly reduced the number of necrotic fruit clusters due to leather rot. However, Captec+Topsin+Kocide (the grower’s standard) and Agri-Fos were numerically superior to the other treatments, while Kocide 2000 alone was the least effective. Captec+Topsin+Kocide also reduced leather rot the most in mature berries at both harvests, although all treatments except for Kocide 2000 were statistically equivalent at the first harvest and Agri-Fos, Phostrol, Physpe (both programs), and Aliette at the second harvest. These data show that phosphorous acid fungicides such as Agri-Fos, Phostrol, and ProPhyt have efficacy against leather rot and worked as well as Aliette. At similar phosphorous acid equivalents, Agri-Fos and Phostrol appeared to be working somewhat better than ProPhyt. Physpe (trade name: Vacciplant), an experimental plant defense booster, seems to be the most promising alternative product for control of leather rot. Treatments that provided significant control of berry cap necrosis of the berries were Physpe (long program), Aliette, Phostrol, and Captec+Topsin+Kocide. The latter program was numerically the most effective. Differences between treatments were not significant at the second harvest. Roughly the same trends were seen in berry cap necrosis and in leaf scorch control, which makes sense since the leaf scorch fungus was responsible for most of the berry capnecrosis. For both berry cap necrosis and leaf scorch control, Physpe (5 sprays) tended to be somewhat better than Physpe (4 sprays). Angular leaf spot levels on the leaves were low and differences between the treatments were not significant. Not much fruit rot occurred before harvest, so berries were incubated after harvest. The most effective product was V10135, a new experimental fungicide from Valent with known Botrytis activity. The phosphorous acid fungicides did not provide significant control of post-harvest Botrytis gray mold. Differences at the second harvest were not significant. The Agri-Fos, Phostrol, and Prophyt treatments exhibited phytotoxicity as a slight burning of the leaf tips but did not affect the fruit. No other treatments showed phytotoxicity symptoms.
Table 1. Fungicide efficacy against leather rot in ‘Red Chief’ strawberries in Onondaga, MI, 2005.
Application timingz / Leather rotyNecrotic fruit clusters / Fruit harvest 1 / Fruit harvest 2
Treatment, rate/A / Number
per plot / Control [%]w / Incidence (%) / Control [%] / Incidence (%) / Control [%]
Untreated
/ 24 / ax / 21 / abv / 35u / atKocide 2000 2 lb...... / 1, 2, 3, 4 / 11 / b / [54] / 28 / a / [-33] / 26 / abc / [26]
V 10135 0.4 lb...... / 1, 2, 3, 4 / 9 / bc / [63] / 9 / bcd / [57] / 21 / abc / [40]
V 10135 0.6 lb...... / 1, 2, 3, 4 / 8 / bc / [67] / 11 / bcd / [48] / 23 / ab / [34]
ProPhyt 3.6 pt...... / 1, 2, 3, 4 / 7 / bc / [71] / 14 / abcd / [33] / 18 / abc / [49]
Aliette WDG 4 lb...... / 1, 2, 3, 4 / 4 / bc / [83] / 9 / bcd / [57] / 13 / abcd / [63]
Physpe 14.4 fl oz...... / 1, 2, 3, 4 / 4 / bc / [83] / 5 / cd / [76] / 13 / bcd / [63]
Physpe 14.4 fl oz...... / 1, 2, 3, 4, 5 / 3 / bc / [88] / 5 / cd / [76] / 9 / cd / [74]
Phostrol 3.5 pt...... / 1, 2, 3, 4 / 2 / bc / [92] / 13 / bcd / [38] / 11 / bcd / [69]
Agri-Fos 4.5 pt...... / 1, 2, 3, 4 / 1 / c / [96] / 14 / abcd / [33] / 8 / bcd / [77]
Captec 4L 2.5qt
+Topsin M 70WP 1 lb
+Kocide 2000 2.75 lb...... / 1, 2, 3, 4 / 1 / c / [96] / 4 / d / [81] / 4 / d / [89]
zSpray dates: 1 =3 May (green foliage), 2 = 24 May (bloom), 3 = 2 Jun (bloom + green fruit), 4 = 14 Jun (green and red fruit,
1 day pre-harvest) and 5 = 21 Jun (red fruit, 1 day pre-harvest).
yBlock 3 was dropped from the data due to uneven distribution of disease.
xColumn means followed by the same letter are not significantly different according to Fisher’s Protected LSD test (P0.05).
wBracketed values denote percent control relative to the untreated check.
vColumn means followed by the same letter are not significantly different according to Fisher’s Protected LSD test (P0.09).
uValues shown are actual means; statistical analysis was performed on square root-transformed data.
tColumn means followed by the same letter are not significantly different according to Fisher’s Protected LSD test (P0.06).
Table 2. Fungicide efficacy against berry cap necrosis in ‘Red Chief’ strawberries in Onondaga, MI, 2005.
Berry cap necrosisyTreatment, rate/A / Application timingz / Harvest 1 / Harvest 2
Incidence (%) / Control [%]w / Incidence [%]
Untreated
/ 43.3 / a / 31.7 / nsV 10135 0.6 lb...... / 1, 2, 3, 4 / 41.7 / abx / [4] / 30.0
ProPhyt 3.6 pt...... / 1, 2, 3, 4 / 36.7 / abc / [15] / 40.0
Physpe 14.4 fl oz...... / 1, 2, 3, 4 / 31.7 / abcd / [27] / 33.3
Kocide 2000 2 lb...... / 1, 2, 3, 4 / 31.7 / abcd / [27] / 16.7
V 10135 0.4 lb...... / 1, 2, 3, 4 / 30.0 / abcde / [31] / 35.0
Agri-Fos 4.5 pt...... / 1, 2, 3, 4 / 30.0 / abcde / [31] / 23.3
Physpe 14.4 fl oz...... / 1, 2, 3, 4, 5 / 23.3 / bcde / [46] / 20.0
Aliette WDG 4 lb...... / 1, 2, 3, 4 / 16.7 / de / [61] / 11.7
Phostrol 3.5 pt...... / 1, 2, 3, 4 / 15.0 / de / [65] / 16.7
Captec 4L 2.5qt
+Topsin M 70WP 1 lb
+Kocide 2000 2.75 lb...... / 1, 2, 3, 4 / 11.7 / e / [73] / 23.3
zSpray dates: 1 =3 May (green foliage), 2 = 24 May (bloom), 3 = 2 Jun (bloom + green fruit), 4 = 14 Jun (green and red fruit, 1 day pre-harvest) and 5 = 21 Jun (red fruit, 1 day pre-harvest).
yCalyx necrosis included browning due to the leaf scorch fungus and blackening due to angular leaf spot bacteria.
xColumn means followed by the same letter are not significantly different according to Fisher’s Protected LSD test (P0.05); ns = not significant.
wBracketed values denote percent control relative to the untreated control.