Patrick D. Colyer, Boyd Padgett, William D. Caldwell and Philip R. Vernon
In the past few years there has been a resurgence of a boll rot often referred to as boll dangle, Phomopsis boll rot, atypical boll shed or vascular cavitation. This boll rot has been present at low levels for many years but has become more severe in the past eight years. Researchers described a similar boll rot on cotton in Louisiana in 1963, which they attributed to the fungus Phomopsis. Although Phomopsis is frequently isolated from infected bolls, its role in the etiology of the disease is uncertain.
Boll dangle is characterized by the death of small (thumbnail size) bolls that remain attached to the plant, often suspended by a strand of bark below the node, hence the name (Figure 1). A lesion often forms around the base of the peduncle of the boll and may extend several inches down the stem (Figure 2). This boll rot is different from typical boll rot because it is not restricted to the denser lower canopy of the plant and affects mainly young bolls. The disease is most evident in late July or August. The epidemiology and full impact of this condition on cotton development and yield are not fully understood.
Tests were conducted to document the incidence of boll dangle on cotton varieties entered in the statewide cotton variety tests from 1997 to 1999. Over the three-year period, 81 early-maturing and 107 medium-maturing varieties were evaluated in the trials conducted at the Dean Lee, Macon Ridge, Northeast and Red River research stations. Boll dangle incidence (number of affected bolls per plant) was measured by counting the number and position of symptomatic bolls on 30 plants of each variety (10 plants per replicate) in late July or early August of each season.
Incidence of boll dangle was highest in the medium-maturing varieties at all locations. Across all varieties, boll dangle incidence was highest each year at Macon Ridge and Northeast. Nearly every variety was susceptible to boll dangle, but the number of affected bolls rarely exceeded two per plant. Each year in each test, 70 percent of the affected bolls were located in the first position (Figure 3). Severity of boll dangle was generally greater on plants with heavy boll loads.
Because the varieties in the test changed annually, it was impossible to evaluate all varieties across all three years. In the early-maturing varieties, Suregrow 501 had the highest incidence of boll dangle all three years. Other varieties heavily affected in the early test were Phytogen PS355, Deltapine 5415 and Stoneville 474. In the medium-maturity test, Suregrow 248 had the highest incidence of boll dangle in 1997 and 1998. Other varieties with a high incidence of boll dangle in the medium test were ACSI EXP0805, HCR9310, Deltapine 90B, Deltapine 9775, Deltapine 5690RR and Deltapine 675.
Cotton Development, Yield
In 1999 and 2000, additional studies assessed the effect of boll dangle on cotton development and yield. Researchers identified four fields that exhibited a high incidence of boll dangle. Three were on the Macon Ridge Research Station and one was in Madison Parish. In 1999, the fields monitored were planted in NuCOTN 33B (Macon Ridge) and Deltapine 90RR (Madison Parish), and in 2000, Maxxa (Macon Ridge) and Phytogen 355 (Macon Ridge). In mid-July, 50 pairs of plants in each field were marked for future reference by securing plastic tags around the main stem. Each pair consisted of a plant with boll dangle symptoms and a plant without symptoms. Paired plants were adjacent or in close proximity within the same row. The tagging date, the condition of the plant (affected or not affected) and the pair number (1-50) were recorded on each tag.
At tagging, the number of harvestable bolls (24 days old or older) and boll dangle severity (number of affected bolls per plant) were assessed. The location of each affected boll was identified by node number and fruiting position. Boll dangle severity was re-assessed every seven to 14 days until harvest. During mid-September, plants were chemically defoliated when 60 percent of the bolls were open. After defoliation, each plant was hand picked, and seedcotton from vegetative, first, second and all other boll positions was recorded. (Figure 3)
In 1999, initial boll dangle incidence was similar in both fields, but, by the end of the season, incidence was highest in Deltapine 90RR (4.64 affected bolls per plant) (Table 1). Most of the affected bolls (69 percent for NuCotn 33B and 56 percent for Deltapine 90RR) were at first position sites. Affected plants produced more harvestable bolls and seedcotton than non-affected plants in both varieties. Most seedcotton per plant for affected and healthy plants was produced from first and second position bolls, followed by vegetative bolls.
Results on cotton development and yield from 2000 were similar to those in 1999. Phytogen 355 had more infected bolls than Maxxa (Table 1). Most affected bolls in both varieties were located at first position sites (54 percent for Phytogen 355 and 72 percent for Maxxa), followed by second position sites. The number of harvestable bolls was similar for affected and non-affected plants in Phytogen 355 and Maxxa. Seedcotton yields did not differ among affected and non-affected plants of Maxxa; however, the affected plants of Phytogen 355 produced more seedcotton than did non-affected plants.
Results from these two studies indicate that even though the incidence of boll dangle has been increasing, the disease did not affect cotton development or yield adversely. In statewide variety trials, the incidence of boll dangle rarely exceeded two bolls per plant. In monitored fields with infestations with select varieties, more harvestable bolls were produced on affected plants in both varieties evaluated in 1999 and did not differ among affected or non-affected plants in 2000. (This article was published in the summer 2000 issue of Louisiana Agriculture.)
Patrick D. Colyer, Professor, Red River Research Station; Boyd Padgett, Associate Professor, Macon Ridge Research Station, Winnsboro, La.; William D. Caldwell, Professor, and Philip R. Vernon, Research Associate, Red River Research Station, Bossier City, La.
(This article was published in the summer 2000 issue of Louisiana Agriculture.)