Common Rust in Maize is a major risk for growers in the upcoming season

Last summer season, severe rust infections in crops around the Central Darling Downs were estimated to have caused grain yield losses of 25% or more in some maize corn varieties planted in the late December-early January planting window.

During these events, staff from Black Earth Agronomy forwarded samples of maize corn and sweet corn infested with rust to Dr Sue Thompson, Plant Pathologist with the Centre for Crop Health at USQ. Sue passed the samples on to Roger Shivas at the BRIP Herbarium in Brisbane to confirm the identification of the causal rust species.

All the samples have been identified as common rust, Puccinia sorghi. What the herbarium can't determine is the race of the Puccinia sorghi. However Black Earth staff have observed severe rust infections in varieties of sweet corn in North Queensland which were previously tolerant of the disease. This would indicate a change in the pathogen.

Common rust in corn is caused by the fungus, Puccinia sorghi, which does not infect sorghum as the name would imply.

The optimum temperature for the fungal spores to germinate and infect corn leaves is in the range 15-25 degrees C combined with high humidity or surface moisture for 4 hours or more from dew or rainfall. However the fungus can infect and sporulate (produce secondary spores) from 4-32 degrees C. Consequently the disease is more likely to be a problem in cool, moist conditions which are more likely to occur in the spring and autumn corn crops on the Downs and Lockyer Valley and during the winter months in the warmer areas along the Queensland coast (e.g. Bowen/Burdekin).

Common rust is not a frequent disease problem in maize corn in the main corn belt in the U.S. because resistant or tolerant varieties are grown and because the disease does not survive during the cold winter months. The spores must be carried by the wind from the warmer regions of the USA and Mexico where both maize corn and native corn types persist over winter, before infection can occur. This often happens too late in the corn growing season to have any effect on yield.

My reading of the literature on the net would suggest that common rust is a significant issue in maize corn in Hawaii and parts of South Africa and Argentina where the spores can survive between the seasons on successive plantings of corn or on alternative host plants including creeping oxalis (Oxalis corniculata) or other species of Oxalis which are common broadleaf weeds in gardens and parklands. Yield losses of upto 35% have been reported where severe epidemics have occured in susceptible corn varieties.

This would suggest that we will have to be far more vigilent in scouting  maize and sweet corn crops to monitor the levels of rust which may appear if the conditions are favourable. Our experience has shown that the rust can proliferate very rapidly in some varieties of maize corn and sweet corn to the extent that a single application of the fungicide Tilt 250 (propiconazole) at 10 true leaves (V10) could not contain the disease outbreak. Tilt 250 has a permit to control northern leaf blight in maize, sweet corn and popcorn but not common rust. Jerald Pataky from Illinois reported in 2001 that "over a 4 week period, one rust pustule can produce 5,000 spores. Thus, a million spores can be produced on the bottom 3 to four leaves of a corn plant that has as few as 50 to 70 pustules per leaf (about 5% severity)." Apparently Tilt 250 does limit the mycelium growth of rust in the plant leaf tissues but does not kill the spores or stop the spores from germinating on new leaf growth. In practical terms, this means that a spray with propiconazole might be warranted as early as V5 (5 true leaves) if the conditions are favourable and the disease is seen to be moving up the crop canopy.

Given that there would appear to be change in the pathogenicity of common rust to current commercial varieties of maize and sweet corn in Queensland, it would be beneficial if corn and sweet corn growers could obtain Permits for products other than just propiconazole to control the common rust outbreak. Their counterparts in the USA have access to a wide range of Triazoles (e.g. propiconazole, tebuconazole, cyproconazole, epoxiconazole) and Strobilurins (e.g.  azoxystrobin, pyrachostrobin) often combined in a single product label mix to improve efficacy and to reduce the likelihood of the development of fungicide resistance.

Products such as azoxystrobin (Amistar) do kill the spores which can significantly slow the rate of development of a rust outbreak, particularly if used in combination with a products from the Triazole family.

Written by

Graham Boulton

Black Earth Agronomy

0429 063907

The information provided above is based on experience and knowledge developed while operating as an Agronomist on the Darling Downs and the Burdekin. The opinions contained within this post are entire that, and may not apply to a grower's specific circumstance.  We recommend consulting your own agronomist to ensure best performance on your own farm.

 

Mango Scale and Pink Wax Scale IPM in the Burdekin

Developing and implementing an Integrated Pest Management (IPM) plan for Mango scale and Pink Wax scale in the Burdekin and northern mango production areas is key to controlling these pests. These pests cause minor yet frequent damage to mangoes and other crops, and in badly infested produce, this damage can effect most of the internal fruit resulting in down grading in class and return on crop.

mango-scale-damage

Figure 1.0: damage cause by mango scale, Holmes, R, et al. n.d.

female-false-mango-scale pink-wax-scale

Figure 2.0: Female False mango scale on the left and Pink Wax scale on the right showing the difference, Northern Territory Government Department of Resources, 2010

Mango scale is actually two different scales; False Mango Scale and White Mango Scale.  False mango scale appear yellowish with scale covers that are flat, white and in a shape that looks similar to a pear. White Mango scale females are described as being red with scale coverings that are flat, white and circular with a black oval shaped caste skin (Northern Territory Government Department of Resources, 2010). The males of both species are white, rectangular in shape with two or three distinct ridges and are usually clustered round a single female.  Pink Wax Scale adults are covered in a pinkish to brown wax substance, are shaped like a globe and are only found on the midrib of leaves (see figure 2).

The scale are sucking pest which feed on the sap from the plant and congregate on parts of the plant that are high in sap, new shoots, leaves (see figure 1), flower panicles, and the fruit stalks/surface. The main component of the sap that the scale feeds on is the protein while other components of the sap are excreted. This scale excretion is high in sugars which provides an ideal medium for the growth of black sooty mould (Department of Agriculture and Fisheries, 2012).

black-sooty-mould-on-mangos black-sooty-mould-on-mangos-2

Figure 2.1: Black Sooty mould on mangos, photos courtesy Owens G, 2016.

This mould (see figure 2.1), in sufficient quantities, can impact on the photosynthetic capacity of the plant, which then impacts on the ability of the plant to yield, thus a loss of production occurs. Other pests, such as mango leaf hopper, also produce the high sugar content excretion and have been seen to turn entire orchards black with sooty mould, resulting in major impacts on the plants’ ability to photosynthesise.

Integrated Pest Management

A report by Farm Biosecurity (2013) states that Integrated Pest Management (IPM) combines the use of biological, cultural and chemical practices to control insect pests in agricultural production.

Scale have a joined life cycle where, from eggs to first moult, the sex is not defined however roughly 80% of crawlers will become males and follow one path of the lifecycle, while the remaining 20% will become females and follow a different lifestyle path (see figure 2.2). To gain the best control of the scale’s lifecycle, the crawler stage of the cycle should be targeted through the implementation of an Integrated Pest Management (IPM) strategy. This is particularly important when using the chemical component of the overall approach when managing scale.

scale-life-cycle-holmes-r-et-al

 Figure 2.1: Scale life cycle, Holmes, R, et al. n.d.

Biological control

There are two main types of biological agents that target scale, parasites and predators. The biological agents are often present in back ground levels where the pests occur and are in a type of balance where their population size rises and falls with the availability of their associated pests. Often is the case where these biological agents are susceptible to similar chemistry that is used to control other pests in crop which can lead to population explosions of the pest. A delay is often occurs in these cases where the pest population rises quickly and the beneficial insects are overwhelmed with numbers and can’t keep up.  This is where release of beneficial insects or crop refuges allows a healthy population of beneficial insects to be maintained.

A parasite, Anicetus beneficus, was introduced in 1977 and was shown to be very effective in controlling the pink wax scale where no disruptive pesticides were used. A good indication of whether or not these parasites are present and actively feeding, will be the presence of a small circular hole where it has emerged on the scale.

Releasing parasitised scale in an orchard can boost existing background populations. In addition to this, a number of other commonly found parasitic wasps, including Metapyhcus varis, Scutellista caerulea and Coccophagus ceroplastae, can also be used to feed on scale. It is often worthwhile to leave a tree infested with Pink Wax scale as it acts to increase the parasite numbers in the orchard to the point where spraying is seldom necessary.

There are also a number of common predators that actively feed on scale including the caterpillar (Common, 1990), lacewings (mallada spp), and multiple types of ladybeetles, such as the Black Lady Beetle (Rhyzobius ventralis), Cryptolaemius (Cryptolaemus montrouzieri) and Common spotted ladybird (Hamonia confromis).

Cultural control

Mango scales prefer shaded environment and this can be managed by maintaining an open tree structure when the crop is being pruned following the harvest. In order to remove residual scale populations on plant material, it is recommended to open up the canopy to allow for improved penetration from sprays and access to more light. This will reduce favourable conditions for scale to continue to increase in the off season (see figure 3.0).

uncleared-mango-canopy cleared-mango-canopy cleared-canopy-mangos-2

Figure 3.0 difference between not cleaned out and cleaned out internal canopy of trees, Photos courtesy of Owens G 2016.

There have also been some observed cases of a significant increase in the population of mango scale shortly after the dust in clay soils has settled on the leaves of the tree. This can be mitigated by the use of wind breaks, or by minimising exposed soil surfaces on the farm, such as tracks and road ways.

Chemical control

Chemical options are often used to control mango scale and, when used in conjunction with both cultural and biological control methods, can provide more effective control of the target while still protecting the beneficial insects. There are three chemical control program options that are currently in use, and example of this can be found on the Movento Users Guide for Mangoes (Bayer Crop Science):

  1. with no IPM and using broad spectrum insecticides,
  2. using some IPM and broad spectrum insecticides, and
  3. using full IPM and selective insecticides to protect beneficial insects in the orchard.

Selection of the right chemical is as important as selecting the right time in the pest’s life cycle to apply the chemical to achieve the desired outcome. Products such as Movento when used with an IPM program provide good control of scale insects in mangoes post-harvest and after flowering.

Summary

Mango scale and Pink Wax scale are minor yet frequent pests in the mango industry across Australia, and cause significant damage to the plant and the volume of the crop. IPM helps to provide good control on individual farms, however when embraced by the agricultural area, it can provide more widespread control and prevent pest incursions from neighbouring farms.  Often the scale in these orchards have been indirectly selected for resistance through continued use of chemicals. This area-wide approach can help to generate better control of scale insects, protect current chemistry, as well as prevent chemical failures in the future.

Written by Andrew Owens

Mobile: 0400 989 253

Black Earth Agronomy

References

Farm Biosecurity, 2013, ‘What is integrated pest management?’ viewed 15th September 2016, http://www.farmbiosecurity.com.au/what-is-integrated-pest-management/

Bugs for Bugs, 2015, ‘Cryptolaemus’, viewed on the 15th September 2016, https://bugsforbugs.com.au/product/cryptolaemus/

Common, I, F, B, 1990, ‘Moths of Australia’, Brown Prior Anderson Pty Ltd, Burwood, Victoria, viewed on the 14th September 2016, Ihttps://books.google.com.au/books?id=magzbmvdRvQC&pg=PA70&lpg=PA70&dq=Catoblemma+dubia&source=bl&ots=5LqM1OfkNM&sig=ap4eHZUACO2LrGRaMHfBQqdpwsM&hl=en&sa=X&ved=0ahUKEwiazJzaypXPAhVImJQKHXSEADUQ6AEIIDAB#v=onepage&q=Catoblemma%20dubia&f=false

Salvan, T, 2016, ‘Organic mango production: strategies and methods; Pest and disease management’, viewed on the 20th September 2016, https://www.agric.wa.gov.au/mangoes/organic-mango-production-strategies-and-methods?page=0%2C5

Pinese, B, 2006, ‘Integrated Pest Management (IPM) in Australian Mangoes’, viewed on the 16th of September 2016, http://aciar.gov.au/files/node/774/03%20Pinese%20IPM%20in%20Australia.pdf

Holmes, R, Weinert, M, Wittenberg, L, Pinese, B, Freebairn, C, Bally, I, Frazer, M, n.d. ‘Managing Mango Scale’, viewed on the 14th September 2016, https://www.nt.gov.au/__data/assets/pdf_file/0018/228015/mango_scale_management_poster.pdf

Department of Agriculture and Fisheries, 2012, ‘Pink wax scale’, viewed on the 14th of September 2016, https://www.daf.qld.gov.au/plants/fruit-and-vegetables/a-z-list-of-horticultural-insect-pests/pink-wax-scale#

Bugs for Bugs, 2012, ‘Pink Wax Scale’, viewed on the 15th September 2016, https://bugsforbugs.com.au/2012/03/pink-wax-scale/

Northern Territory Government Department of Resources, 2010, ‘Field Guide to Pest, Beneficials, Diseases and Disorders of Mangoes’, viewed on the 20th of September 2016, https://dpif.nt.gov.au/__data/assets/pdf_file/0006/227832/mango_field_guide.pdf

Bayer Crop Science, n.d.  ‘Users’ Guide for mangoes; How to get the best out of movento in mangoes’, viewed on the 20th September 2016, https://www.google.com.au/search?q=movento+user+guide+for+mangoes&sourceid=ie7&rls=com.microsoft:en-AU:IE-Address&ie=&oe=&gfe_rd=cr&ei=eTTrV_bIG6rM8getmai4CQ

The information provided above is based on experience and knowledge developed on farm and as an agronomist. The opinions contained within this post are entire that, and may not apply to a grower's specific circumstance.  We recommend consulting your own agronomist to ensure best performance on your own farm.

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