All the edible alliums grown from seed are similar in having slow germination and emergence rates and low relative growth rates in comparison with other crop species. This, along with their erect, narrow leaves, makes them vulnerable to suppression by faster-growing weeds over a long period of early growth (see Chapter 5, ‘Effects of weed competition’). If they are planted as bulbs (sets or cloves) rather than seeds, the larger propagule makes for easier and faster crop establishment.
In those species that can form bulbs and inflorescences there are striking similarities in the environmental control of these two developmental pathways. Long photoperiods stimulate bulbing, the later stages of flowering when the scape is elongating and sometimes the earlier stage of inflorescence initiation. Light spectral quality as characterized by red:far-red (R:FR) ratio modifies the photoperiodic effectiveness of light in stimulating bulbing. The lower the R:FR in an inductive photoperiod the faster the bulbing, and the minimum photoperiod for bulb induction increases as R:FR increases. Direct R:FR effects on flowering have not been reported, but high plant densities, which cause low R:FR under the leaf canopy, can increase flowering (percentage of plants bolting) (Bosch- Serra and Domingo-Olivé, 1999), indicating there may be a promotive effect of low R:FR on inflorescence induction or scape elongation.
With stored bulbs, and sometimes growing plants, a period of cool temperatures (5–13°C) can accelerate subsequent bulbing. Extended periods at such temperatures are necessary for inflorescence induction (i.e. for vernalization). Periods of warm temperature (25–35°C) applied to bulbs of onion, shallot and A.⳯ wakegi immediately after cool treatment nullify bulbing acceleration. Extended periods at such temperatures reverse inflorescence induction and have a devernalizing effect (see Fig. 4.39). Examples where flowering has diverted into bulbing are seen at many points along the route of inflorescence development (see Fig. 4.35; Kamenetsky et al., 2004). First, there are the withered young inflorescences seemingly squashed by an adjacent bulb
(see Fig. 4.38), then there are examples of one or a few bulbs forming on partially elongated scapes and finally there are fully emerged inflorescences carrying numerous bulbils that have developed by suppressing the developing florets (see Fig. 2.18e, f, g and Fig. 4.41) or after deliberately removing the florets. It seems likely that generations of selection for bulbs, which constitute an easily harvested and stored source of food and an easily cultivated propagule, have favoured bulbing in garlic and shallots to the extent that flowering and seed production is eliminated in some strains, and in other cultivars flowering can be induced only by a very precise sequence of environments each specific to a particular phase of floral development (Kamenetsky et al., 2004; Esnault et al., 2005).
There seems to be fine balance between the path of inflorescence develop- ment and bulb development, with warmer temperatures tending to favour the latter, although this depends on the stage of inflorescence development, as after the stage of floret development warm conditions can favour rapid progress to anthesis and seed ripening. What constitutes a long photoperiod and low temperature will vary with cultivar and the climate to which it is adapted, so we can only describe trends in bulbing and flowering responses to these variables. The absolute values of photoperiod and temperature needed to progress flowering or bulbing at a particular stage of development will be specific to a particular cv. and similar for other cvs adapted to the same or similar region.
In view of these similarities in the control of flowering and bulbing and the easy switching between the two developmental paths, it is interesting to consider how this connection evolved. It seems likely that bulbing can give a plant a better chance of survival if climatic conditions deteriorate such that flowering will not result in seed production. It also allows slow-growing plants like alliums to reach a size large enough for flowering in environments with a short growing season. Is it possible that bulbing evolved as a deviant or diverted flowering response? Do the same genes control the similar responses to environment of both developmental paths?
5
© J.L. Brewster 2008. Onions and other Vegetable Alliums, 171
2nd Edition (J.L. Brewster)
INTERACTIONS WITH
OTHER
ORGANISMS: WEEDS, PESTS,
DISEASES AND
SYMBIONTS
INTRODUCTION
The grower of vegetable alliums does not operate in a sterile environment, and his crops may be assailed by a variety of diseases and pests which may slow growth, lower yield and cause damage and disfigurement that renders them unmarketable. Allium crops are also particularly susceptible to competition and suppression from weeds, and the reasons for this are outlined.
The main pests and disease-causing organisms and the symptoms they can cause are briefly described. The intention is to show how scientific knowledge of weed, pest and disease biology underlies rational crop protection. Attention is also paid to the interrelations between different classes of noxious organisms. For example, allium viral diseases are vectored by pest insects and mites, pest damage can predispose plants to infection by disease-causing bacteria and fungi, and weeds often act as alternative hosts for such bacteria and fungi. This chapter ends with a section on the mycorrhizal fungi of allium roots where, refreshingly, we see an example of fungi that can be of direct benefit to the plants. Symptoms of a number of the important pests and diseases are illustrated in Plates 2 to 9.
There are several publications giving excellent colour illustrations of allium pests and diseases, along with drawings and details helpful for their identification, sometimes presented as diagnostic keys. These include the book by Schwartz and Mohan (2008) and the CD-ROMs of Maude and Ellis (2001) and BCPC (2003). Excellent illustrations, along with details of biology and control methods, are freely available at numerous web sites, particularly those of many US land-grant universities, in particular Colorado State University, which includes a link to many other ‘onion and garlic’ web sites (http://www. colostate.edu/orgs/VegNet/vegnet/onions.html) and the University of California, which gives a comprehensive coverage of weeds, pests and diseases (http://www. ipm.ucdavis.edu/PMG/crops-agriculture.html). The research literature on pests, diseases and weeds was reviewed in Rabinowitch and Brewster (1990b), and more recent reviews of many relevant topics may be found in Rabinowitch and
Currah (2002). These sources have been drawn on in the preparation of this chapter.
The application of new technologies to plant pathology, in particular, has resulted in rapid progress in recent years. Diagnostics are being made faster and more precise by new molecular methods that identify pathogens by their DNA or RNA sequences, and also by the development of rapid immunological tests for pathogens (Ward et al., 2004). With these techniques, rapid and accurate diagnoses can be made without the need for highly trained specialists. These methods are also opening up new fields of study relevant to plant disease. For example, it is becoming possible to investigate the interactions between mixed species of microbes, including pathogens and their antagonists, on leaf surfaces or close to roots. In addition, the rapid detection and quantification of airborne pathogen spores, along with automated electronic monitoring of the microclimate in crops, can input real-time data into weather-based models for predicting pathogen infection and epidemic development. These predictions enable fungicides to be applied only when they are beneficial.
In developed economies vegetable producers are required by supermarket buyers to supply high-quality, blemish-free produce and, at the same time, there is public and political pressure to reduce pesticide usage. Needless to say, these two requirements are often in conflict and can be reconciled only by developing more sophisticated methods of crop protection based on scientific knowledge. As an example, the models for forecasting required fungicide application, mentioned above, can reduce unnecessary spraying. Such technologies are incorporated within programmes of ‘Integrated Pest Management’ (IPM), which aim to control weeds, pests and diseases by using pesticides in conjunction with other aspects of crop protection, including cultural methods, resistant varieties and the encouragement of beneficial organisms.
Surveys of actual pesticide use on commercial bulb onion crops in the UK indicate a modest increase in the amount of pesticides applied in recent years (total kg/ha active ingredients (AI)) but a large increase in the number of applications, particularly of herbicides and fungicides (Thomas, 2003). Between 1986 and 1999 the mean number of herbicide sprays per crop increased from around five to nine to ten, but the weight of AIs applied increased by only 19% (Grundy et al., 2003). The reason for this was a continuing trend towards more ‘repeat low-dose’ herbicide treatments. In the same period the mean number of fungicide sprays increased from about three to five to six (Thomas, 2003). Improvements in application technology – whereby less pesticide gets wasted on non-target surfaces – and better chemicals, which are effective at lower concentration, are both tending to limit increases in amounts applied despite increases in the number of applications. In contrast, there is a growing market for ‘organic’ vegetables grown without using chemical pesticides. The main limitation for large-scale ‘organic’ production of allium vegetables is the need for many hours of hand-weeding to produce satisfactory crops (Melander and Rassmusen, 2001).
Environmental concerns have led to increased governmental regulation of pesticide use in recent years. As a result, the costs have increased of proving the safety and efficacy of pesticides and of registering them for use on specified crops. In addition, there has been a requirement for older pesticides to be tested to modern standards and re-registered if their use is to continue to be permitted. It is now uneconomic for manufacturers to register pesticides for use on anything other than the major arable crops where the areas planted generate sufficient demand to recover registration costs. In these terms the allium vegetables, in common with other vegetables, are minor crops. Various schemes have been instigated in different jurisdictions whereby growers themselves, or a combination of growers and public bodies, finance the testing and registration of pesticides for use on these ‘minor crops’. In the UK, uses for minor crops are granted by ‘Specific off-Label Approvals’ (SOLAs), and the costs of testing and registering for these are borne out of a research and development levy on growers. By this process it is possible to provide growers with a range of appropriate pesticides that allows them to continue production of vegetable crops economically (Chapman, 2000; Knott, 2005).