Over the past five years I have been exploring various potential applications of Normalized Difference Vegetation Index (NDVI) satellite imagery for use within a broad range of agronomic practices.   Combined with infield monitoring, this imagery can be used to assess and identify causes of variation within a field, including varying soil types, soil constraints, plant available water capacity (PAWC), plant stand quality, poor drainage, plant health and disease. The usefulness tool is compounded by the availability of historic imagery of fields dating back as early as 2000.

The purpose of this article is to outline various considerations I explored as to how NDVI satellite imagery can be used for defoliation timing in cotton.  I will discuss the practical application of using this technology, as well as benefits and limitations I have encountered since adopting this approach as part of my field management. Recently I have been using imagery software provided by Satamap Global, however other online imagery services, such as Data Farming, are equally as useful.

SVI/NDVI Application

Satamap Global provides imagery from Earth observation satellites such as Landsat 8 (including previous version data), MODIS, and Sentinel 2. It offers this imagery in the form of Satamap Vegetation Index (SVI) images, an index similar to NDVI which considers the different reflective properties of green, red and near infrared bands to give a picture of relative crop variation and development.

Image 1 shows a cotton field very close to its first defoliation. The pink area running through the middle of the field shows where an additional irrigation occurred, which correlated well with having the highest number of nodes above cracked boll (nacb) in the field. The colour scale shown in the bottom left corner reflects relative nacb across the field to some extent.

Image 1

Using SVI imagery, I identified some correlation between different fields and the colour scale/nacb on the same day but this did not always hold true. Occasionally there would be a discrepancy of up to 4 nacb when comparing colour scale/nacb across fields. Given this, I would be particularly cautious when using this between fields without physically inspecting both fields. The colour scale/ nacb correlation was particularly poor when comparing an irrigated field to a dryland field. Dryland fields were typically lower on the colour scale despite having higher nacb.  

Another risk associated with using SVI to assess defoliation timing is that it is representative of vegetative growth, not crop quality or maturity. This means that regrowth in a field can show up higher in the colour scale. This is particularly true for refuge cotton due to its higher levels of vegetation, freshness of growth and regrowth. This year, weather conditions caused previously poor yielding areas of cotton to regrow at a higher rate than the better performing cotton, resulting in a higher figure on the colour scale than would otherwise be the case. Knowledge of the field and physical inspections were necessary to effectively manage this issue.

Image 2

Image 2 shows another cotton field close to defoliation. The pink area in this field represents a replanted portion of the field and was 2 to 4 nacb above the blue section. Physical inspection alongside the SVI imagery improved confidence in the decision to split the defoliation application timings, however, in this case there was enough of the field with similar maturity to delay until the entire field was ready for harvest. The pink flecked areas in the blue section were close to the same nacb as the late plant section.

Benefits to crop management

Over the past five years, the SVI imagery has proven to be a valuable tool for crop management for the following reasons:

  • It allowed me to more easily identify where to undertake physical inspections in fields and to determine optimal time for defoliation.
  • It improved my understanding of the size of late maturing areas and how fields were developing as a whole
  • It allowed for greater confidence in decision making and assisted in splitting sections for early defoliation.

Limitations of imagery

While the SVI imagery tool has enhanced crop management for Black Earth in recent years, it is not recommended that this technique be used in isolation. Regrowth and other factors often confused interpretation of images and some discrepancies between colour scale and nacb across fields were also noted.  Given these limitations, field knowledge and physical inspections remain an important element of our company’s agronomic practice.

The information provided in this article is based on experience and knowledge developed over an extended period while operating as an Agronomist on the Darling Downs. The opinions contained within this post are entirely that and may not apply in all circumstances.  We recommend consulting your own agronomist to ensure best performance on your own farm.

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.


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