A perspective on the measurement of time in plant disease epidemiology

The growth and development of plant pathogens and their hosts generally respond strongly to the temperature of their environment. However, most studies of plant pathology record pathogen/host measurements against physical time (e.g. hours or days) rather than thermal time (e.g. degree-days or degree-hours). This confounds the comparison of epidemiological measurements across experiments and limits the value of the scientific literature

SE—Structures and Environment

Two optimal control strategies for carbon dioxide (CO2) enrichment in greenhouse tomato crops have been developed. One uses pure CO2 from a storage tank and the other uses CO2 contained in the exhaust gases of boilers burning natural gas. The optimal strategies maximize the financial margin between crop value and the combined costs of the CO2 used for enrichment and the natural gas used for heating. In this paper, the strategy for optimal control using pure CO2 is presented and compared with strategies used by growers. The optimal strategy for enrichment with exhaust gas derived CO2 is presented in an accompanying paper. Simulations show that at a cost of £0.09 kg-1 for pure CO2 and £0.10 m-3 for natural gas, the optimal enrichment strategy would increase the annual margin of crop value over CO2 and heating costs by £4.6 m-2 (27%) compared to a basic control strategy of enrichment to a concentration of 1000 v.p.m. (parts per million by volume) when ventilators are < 5% open, otherwise enrichment to 350 v.p.m. The optimal CO2 concentration was expressed as an algebraic function of solar radiation, wind speed and ventilator opening angle, and so enabled a quasi-optimal value to be obtained using variables measured by greenhouse environmental controllers. The quasi-optimal equation, with coefficients averaged from simulations over 4 years, gave an increased margin over the basic control strategy of £4.4 m-2 (26%). © 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved

SE—Structures and Environment

Optimized control strategies for carbon dioxide (CO2) enrichment of greenhouse tomato crops using CO2 from the exhaust gases of boilers burning natural gas are presented. In one group of strategies, the heat produced during CO2 generation which exceeds the immediate heat requirement of the greenhouse is stored as hot water and used subsequently for heating. The simulations show that, use of optimal control can increase the financial margin of crop value over the combined expenditure on gas used for CO2 and heating by £2.3 m-2 (11%) when heat is not stored and by £4.9 m-2 (24%) when heat is stored, compared with enriching with CO2 only when heating is required. A 30% increase in gas price reduced the financial margin by 11%, whereas a 30% increase in tomato price increased the margin by 40%. The capacity of the heat store places a limit on the amount of heat that can be stored and consequently on the amount of natural gas that can be burnt and the associated amount of CO2 produced during the day. The optimum size of heat store is 20 × 10-3 m3 per unit greenhouse area. © 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved