Integrated control of powdery mildew and other disease, weed and insect problems in squash in Tonga and Australia

Tonga
A field investigation into control of powdery mildew using fungicides, generally regarded as safe (GRAS) chemicals and inducers of natural resistance was conducted this season from July to October 2005. Powdery mildew severity was relatively mild this season. The most effective fungicide in this trial was Talius (both high and low rates) followed by Punch+Afugan, Sulphur, JMS stylet oil, Actigard and the Control (untreated). Exportable yield in t/ha was highest in Talius treatments with 17.21 (low rate), 15.30(high rate), 13.29 (Punch+Afugan, 12.71 (Rubigan), 12.20 (Sulphur), 11.29 (Actigard), 11.01 (JMS stylet oil) and 10.96 (Control). Sugar content was highest in the Punch+Afugan replicates with 9.81%, Actigard (9.64%), Rubigan (9.40%), JMS stylet oil (9.39%), Talius[high (9.15%)], Sulphur (9.08%), Control (8.70%) and the lowest sugar content was recorded in Talius [low(8.61%)].
Australia
An objective powdery mildew screening procedure was developed using a scanning program to assess % severity. Six cultivars of squash : Kabocha SPS8682, Kabocha SPS3742, Kabocha Amali Delica 2643R3KQJ, Kabocha SPS7764, Kabocha SPS8702, Hybrid Japanese Kurijiman F1were screened for resistance to powdery mildew. All were susceptible with no evidence of partial resistance. In glasshouse trials, whole leaf and detached leaf screening of GRAS and defence elicitors was conducted for control of powdery mildew prior to conducting a field trial. Milsana, Thiovit, Silica, an organic oil product GC3, an oil product Biocover, Actigard, and Ti Tree oil, were used and the best treatments selected for use in a field trial in late 2005. In separate trials optimum concentrations of Milsana and silica for PM control was determined. Milsana and Thiovit were significantly better at controlling powdery mildew in the preliminary trials. For the field trial, an integrated approach was developed in testing efficacy of both Milsana and Thiovit against conventional fungicide treatments. Treatments were: Milsana, Thiovit, Amistar, alternate Milsana + Thiovit, alternate Thiovit + Amistar, alternate Milsana + Thiovit + Amistar, Water and Surfactant, and Commercial Practice. The trial was completed in March 2006 and is currently being analysed.

Aim
To improve field-based crop protection and market quality of squash within a systems framework addressing a) Powdery Mildew, b) silver leaf whitefly, c) virus management, d) weed management and e) grower uptake and market implications.

Objectives
1. Development of integrated field management strategies for control of Powdery Mildew and other fungal diseases.
2. Investigation of sustainable cultural, biological and chemical control strategies to Silver leaf Whitefly infestations
3. Identification and control of the major virus diseases of Kabocha squash
4. Development of effective approaches to suppressing weeds that compete with and limit crop production and profitability for farmers in cropping.
5. Optimization of technological adoption and extension through use of on-farm trials and stakeholder workshops and increase awareness of market implications

Due to the retirement of Dr Pita Taufatofua, Dr Villami Manu, Head of Vaini Research Station, has taken over as Team Leader of the Tongan MAFF research team. In contrast to 2005 when a government strike interrupted trials, the team members have made significant progress in meeting the project objectives in 2006.
Both 2005 and 2006 trials in Tonga and Australia have shown that replacement of fungicides with Generally Regarded as Safe (GRAS) chemicals is just as effective in controlling Powdery Mildew as using traditional fungicides alone. The results indicate that environmentally friendly chemicals can be used in controlling powdery mildew without compromising control. Preliminary testing for fungicide resistance by Powdery Mildew suggests there is no resistance to the current suite of fungicides used in Tonga, but this needs to be verified by further testing with isolates from a greater geographical range.

Weed species and other cucurbit crops were screened as potential alternative hosts for viruses. ELISA tests found that Sonchus oleraceus and Chamaesyce sp. were alternative hosts for ZYMV and WMV. The results indicate that weed control around squash plantations is an important aspect of virus control. Further screening for alternative hosts will be conducted in 2007, as will testing for virus transmission via seed.

A survey of weed species and % cover during succession after ploughing was conducted. An early weed coloniser was Commelina sp. however within 3 weeks Macuna seeds had germinated and become the dominant leafy cover. Trials using Macuna pruriens as a cover crop during the fallow season were also begun in 2006. In July 2007, squash will be either planted through the herbicided Macuna or planted after the Macuna cover crop is ploughed in. Apart from the benefits of weed control, Macuna is expected to increase nitrogen levels in the soil, reducing the input of inorganic fertilizer.

Colonies of the introduced Silver Leaf Whitefly parasitoid Erotmocerus hayati died during early 2006. The parasitoid will be re-introduced from Brisbane in early 2007. Evaluation of safer insecticides that are compatible with IPM, was conducted in 2006. All were effective against SLWF, however data are yet to be analysed.

Ms Sela Tupouniau, the MAFF Plant Pathologist took up a JAF fellowship in January 2007 to study for her PhD. at the University of Sydney.

Under the leadership of Dr Viliami Manu, Head of Vaini Research Station, the Tongan MAFF research team made significant progress in meeting objectives in 2007. The 2007 field trial for controlling powdery mildew on squash using fungicides suggests there is no resistance to currently available fungicides in Tonga. Rotational use of the fungicides is important to prevent resistance development in powdery mildew strains.
The GRAS chemicals tested in Tonga and Australia show consistent effectiveness for the control of powdery mildew of squash. A recommendation will be made on the cost to the Tongan and Australian markets on incorporating GRAS chemicals into the spray schedule, reducing the reliance on fungicides.
A study on the powdery mildew populations in squash in Tonga is underway by Ms Tupouniua as part of her PhD study program. Samples of squash powdery mildew are being tested for their sensitivity against different fungicides along with a microscopic study for taxonomic identification.
Field evaluation of insecticides to control Silverleaf Whitefly was not conclusive because of very low infestation in squash during the experimental season (August 2007). An experiment on biological control of Silverleaf Whitefly in tomato is underway in controlled conditions with the introduced parasite of Eretmocerus hayati from Brisbane CSIRO. Methods for screening alternative hosts of viruses affecting squash production were demonstrated to MAFF personnel and the tests are being conducted.
The experiments for the effectiveness of the cover crop of Macuna pruriens that began in 2006 and again in 2007 has been shown to be beneficial in controlling weed population as well increasing economic returns from squash cultivation.
A survey was conducted in 2007 in squash growing areas for prevalence of insect pest and diseases and weed infestation. Good rainfall and a cool climate in 2007 resulted in increased production of squash, through increased fruit set, despite significant powdery mildew infestation during the season. The same climatic conditions reduced the population of Silverleaf Whitefly, and as a result, lowered the level of infection by virus.